In re Irishmen volunteering for the British Army, there were two largely separate groups:

1) The one more remembered now, the Irish Volunteer dupes of John Redmond who joined en masse after Redmond's September 1914 speech at Woodenbridge, thinking that Redmond's Irish Parliamentary Party had gained a promise of Home Rule (a rather watered-down version of what's now called devolution) if they joined up to support Britain in the war that was to be over by Christmas 1914;
2) The workingmen put out of work by a series of brutally suppressed strikes and management lockouts between 1911 and 1913 and unable to get work to support themselves and their families because they were blacklisted by employers. The 'separation allowance' paid to these men's wives or mothers or children and the pay to soldiers was the largest transfer of wealth to the poor of its era. All good? Not quite: 67% of those killed from the Royal Dublin Fusiliers (which was devastated in Gallipoli and Flanders) came from between the two canals of Dublin.
The better-off Irish were also slaughtered; the rugby and cricket sides joined as 'Old Pals' and were virtually wiped out in Gallipoli.