Upvote:2
Yes. While most of them were Protestants from Ulster, there were eight regiments from the 26 Catholic counties of what we would today call the Irish Republic.
The reason was economic. Members of the English working class found that the pay scale and standard of living in the Army, including in the trenches was an improvement over their civilian lives (although they risked death for this), and men from the Irish countryside were even poorer than English industrial workers.