The Defining Battle in Ulster history?

What is the defining battle in the history of Ulster and the Ulster Protestant identity?
probably the first candidate is the Battle of the Boyne, and it is understandable why many could come to conclusion. It is after all the battle emortablised in myth, legend, artwork and song, not to mention the parades of the Orange Institution across the province and beyond.

the image of William III crossing the Boyne one of the most well
known in Ulster folk lore

The Boyne is certainly an important battle and one that is forever enshrined in Ulster Protestant culture and identity. However I don't think it is the defining battle.
Next candidate you could say is the Siege of Londonderry, it is according to the traditional view the moment the Protestants of Ulster had their backs to the wall so to speak. The moment when they knew nothing but defeat and retreat, seemingly abandoned by their fellow countrymen on the island of Great Britain had to fall back to their plantation citadel with its crumbling walls. Yet despite this the right people were in the right places at the right times and sustained by religious and national pride endured throughout the horrors and hardships to their victory. There is no doubt the symbolism of the siege of Derry is powerful and a worthy candidate.
There is no shortage of battles in Ulster history but today I am of course going to speak of the Somme.
The Somme is to Northern Ireland and Northern Irish identity (or atleast unionist Northern Irish identity) what Gallipoli is the Australia or Vimy Ridge is to Canada.  It is when the young nation finds it feet and secures it future through the blood sacrifice which honored by a grateful empire sees them take their place as equals in a great imperial family.
There are of course some distinct differences between the two dominions and Northern Ireland of course. One is the legal entities and jurisdictions of both Australia and Canada were defined before the Great War. Where as that of Northern Ireland was defined after. Whereas the two battles in regard to the Canada and Australia helped forge a common national identity for Ulster Unionists it defined the right to continue to exist.

Signing of the Covenant 1912

The road to the Sommes role as its defining battle really begins in the late 19th Century with Ulster's opposition to Home Rule, that being self governance for Ireland within the United Kingdom. However it really takes place when the Third Home Rule Bill passes the House of Commons in 1912 due to a Liberal and Irish Parliamentary Party (the moderate Irish nationalist party of the day) coalition. This in turn leads to the events of the Ulster Solemn League and Covenant in which Ulstermen sign in their thousands pledging to resist "the current conspiracy" by "all means necessary." Thousands of women also sign a mirroring covenant pledging to support the men in their struggle. These documents are one of the defining moments in the creation of the modern Ulster Protestant identity. The Covenant is to Ulster what the declaration of Independence is to the United States of America, in that its a founding document, a justification for the country to exist.
The founding of the Ulster Volunteer Force (UVF) and subsequent arming of that force (ironically with guns provided by Germany) in the months that followed seen Ulster folk preparing to make good on their pledge. Whether or not the UVF would have went to war over their own government is a matter of debate and merely academic. For events in Europe overtook the events in Ireland with the outbreak of the First World War in 1914.

The charge of the Ulster Division 1st July 1916

This seen the call to arms the opportunity for Ulster to show its loyalty to the Britain and likewise for Britain to reward that loyalty. A special division was formed from the ranks of the old UVF. The 36th (Ulster) Division. It was the conduct of the 36th Division on that first day of the Somme the 1st of July (which is also the date of the battle of the Boyne in the old calendar) 1916 in that it was the only British unit to reach any of its objectives despite possibly being given the most difficult of tasks in taking the Schwaben Redoubt. This is despite the horrendous slaughter and appalling casualties. Indeed the Division suffered so many casualties that it was withdrawn the next day. Its experience at the somme was one from which it never fully recovered but despite this struggled on through the rest of the war and again proving its metal on other occasions.

Drums from Hamilton Flute Band whose members served in
36th Ulster Division are still emblazoned with the old regiment badge
over 100 years on

It was (at least in the folk memory of Ulster) that in 1921 saw the creation of Northern Ireland as a separate jurisdiction to the Home Rule parliament being set up in Dublin, and the next year when home rule turned to independence following the Anglo-Irish conflict saw Northern Ireland given the ability to determine its own future when under the terms of the Anglo-Irish peace treaty Northern Ireland was given the ability to opt out of the Irish Free State and continue as part of the United Kingdom. An opt out it chose to exercise.
Like other battles the story of the Somme is told in song and ballad, possibly the best being the song "bloody road to the somme" which is relatively accurate in the general detail of the battle in the the 36th Division became a victim of their success and got "cut off with no one to support them" and Mown down by fire from three sides" As John Buchan, 1st Baron Tweedsmuir wrote in History of War:
"North of Theipval the Ulster Division broke through the enemy trenches, passed the crest of the ridge, and reached the point called the Crucifix, in rear of the first German position. For a little while they held the strong Schwaben Redoubt (where), enfiladed on three sides, they went on through successive German lines, and only a remnant came back to tell the tale. Nothing finer was done in the war. The splendid troops drawn from those Volunteers who had banded themselves together for another cause, now shed their blood like water for the liberty of the world."

This song is an excellent as lays out the struggle of the Home Rule debate and evokes characters from the Ulster cycle such as the warrior Cuchulainn the Hound of Ulster and Connal Clean the ancient King of Ulster both figure who defended Ulster from hostile southern Irish forces. It even evokes the flag of Northern Ireland (the red hand and crown).

memorial to the VC's of Ulster's division in the grounds of the
Ulster Tower, Thiepval, France

The valour of Ulster's division is shown in that it received no fewer than nine Victoria Crosses many of which are still celebrated today, including the first to be won in the battle. Pte William McFadzean was posthumously awarded the VC when he threw himself on a grenade that fallen into a trench full of his fellow soldiers, thus symbolising the sacrifice of the thousands of Ulstermen who fell that day.
There were many quotes from Britain's grates regarding Ulster's Division, which perhaps fueled the idea that it was the province's loyalty and sacrifice that ultimately seen it spared the political death sentence of Irish independence.
The King at the time George V said on the day of the armistice
  "I recall the deeds of the 36th (Ulster) Division, which have more than fulfilled the high opinion formed by me on inspecting that force on the eve of its departure for the front. Throughout the long years of struggle, which now so gloriously ended, the men of Ulster have proved how nobly they fight and die"
Winston Churchill often regarded as the greatest Briton of the 20th Century said of their conduct:
"The record of the Thirty-Sixth Division will ever be the pride of Ulster. At Thiepval in the battle of the Somme on 1 July 1916; at Wytschaete on 17 June 1917, in the storming of the Messines Ridge; on the Canal du Nord, in the attack on the Hindenburg Line of 20 November the same year; on 21 March 1918, near Fontaine-les-Clercs, defending their positions long after they were isolated and surrounded by the enemy; and later in the month at Andechy in the days of 'backs to the wall', they acquired a reputation for conduct and devotion deathless in military history of the United Kingdom, and repeatedly signalised in the despatches of the Commander-in-Chief."

dedication tablet in the Ulster Tower with the King's quote

More recently the Ulster historian Richard Doherty recorded: "Whether town dweller or country lad, volunteer or regular, officer or other rank, Catholic or Protestant, the Sons of Ulster knew a comradeship and a trust in adversity that should be a lesson to us all."
But perhaps the greatest and most memorable quote is from someone who apart from this quote is largely forgotten. Wilfred Spender was a staff officer attached to the 36th Division and would go on to help establish the Northern Ireland Civil Service as well as reform the returning members of the former UVF into the Ulster Special Constabulary. He wrote of the battle:
"I am not an Ulsterman but yesterday, the 1st. July, as I followed their amazing attack, I felt that I would rather be an Ulsterman than anything else in the world. My pen cannot describe adequately the hundreds of heroic acts that I witnessed... The Ulster Volunteer Force, from which the division was made, has won a name which equals any in history. Their devotion deserves the gratitude of the British Empire."
It is often said there is no greater honour than to elevated to by others and its certainly true in this case which is perhaps why this quote in particular is remembered and celebrated today. However he goes on to write "The Ulster Division has lost more than half the men who attacked and, in doing so, has sacrificed itself for the Empire which has treated them none too well. Their devotion, which no doubt has helped the advance elsewhere, deserved the gratitude of the British Empire. It is due to the memory of these brave fellows that their beloved Province shall be fairly treated."

Again this feeds into the folk memory that it was through sacrifice that Northern Ireland was allowed to remain in the United Kingdom while the rest of the island broke away. 

Ulster Tower, Thiepval France

Likewise the people of Northern Ireland were quick to ensure their sacrifice would never be forgotten funding the erection of a permanent monument on the battlefield itself. In Fact of the many monuments and memorials on the old battlefields of the Great War the Ulster Tower was the first to be erected.  It was modeled on Helen's Tower on Clandyboyle estate in County Down where Ulster's Division was encamped before deployment and was thus the last Ulster landmark many had seen. Although not the grandest of these noble monuments it set a precedent which all of Britain followed. Hence the battle not only helped shape the British identity of Ulster but helped Ulster shape in part the identity of Britain. 
It is for these why the Somme is possibly the defining moment in Ulster Protestant identity to this day. Furthermore unlike the Glorious Revolution the First World War is still very much a part of British national memonry with the various events and traditions introduced to remember the war such as Remembrance Sunday and the wearing of poppies still a very much a part of British life and identity not just that of Ulster.