Volunteers
The various non-governmental militias formed in Ireland in the period, in reaction to the 1912 Home Rule legislation at Westminster.
Ulster Volunteers — loyalist pro-British militia established in 1912 in Ulster (the northern province of Ireland) to oppose Home Rule and to preserve the Union with Britain. It numbered 100,000 men. It was all but exclusively Protestant. Ulster Volunteers
Irish Volunteers — nationalist militia formed in 1913 in reaction to the above, which sought to guarantee Home Rule and separation from Britain. By late 1914, it numbered 180,000 men. In religion it was predominantly Catholic. Irish volunteers
civil war — by summer of 1914, with two vast militias threatening each other, both armed and training, and the political crisis seemingly insoluble, it was widely believed that Ireland would descend into civil war.
WW1 — upon the outbreak of war in August 1914, Home Rule legislation was shelved "for the duration", and civil war was averted.
but by extension those of Ireland, too.
National Volunteers — Redmond's stance caused a split in the Irish Volunteers. The vast majority, with Redmond, seceded to form the National volunteers, whose members enlisted in their thousands, mostly in the 10th and 16th Divisions. In the novel, Gordie, who fought in the 10th Division at Gallipoli, was amongst these National Volunteers. (The 16th and 36th, Irish nationalist and Ulster loyalist, would eventually fight side-by-side at the Battle of Messines in Belgium, 1917.)
remaining Irish Volunteers — the rump that remained after the split with Redmond, numbering some 11,000, retained the name Irish Volunteers. These Volunteers, far more republican than merely separatist, were clandestinely controlled by the IRB. It was the Dublin brigade of these Volunteers (or those who knew enough to turn up for the event) who fought in the Easter Rising.