The relationship between the UK and Ireland has always been complex and I think it’s simplistic to assume that just because someone supported an Independent Ireland they were therefore necessarily totally anti-British. Why move to England for work if you felt so strongly about it? If your ancestor joined up as an Officer he must have been pretty well motivated towards the UK. He wouldn’t have done that otherwise.
But a good example of the complex relationship between the two countries comes from the 2013 amnesty for Irish Army deserters. In 1939, when Ireland had only been an independent country for 17 years and had a small army, WW2 started. Ireland remained neutral and so their army had no involvement. About one third of that Irish army deserted and joined the British armed forces, to fight against the Germans. So, although they had been fighting for an independent Ireland for hundreds of years, these men were still willing to fight for Britain against the Germans. That’s how they felt.
On their return to Ireland from war in 1945 they were treated as deserters (which they were) and denied Irish pensions and jobs. Many had a difficult time, even though they had shown great bravery. It was 2013 before the Irish Government had a re-think and gave the dwindling numbers of survivors an amnesty and a military pension.
https://www.bbc.co.uk/news/world-europe-22425684To this day some Irish citizens join the British army (as opposed to the Irish Defence Forces). Two I saw interviewed a year or two back said they did so “to get a bit of action” because the Irish Defence Forces normally only ever do peace keeping work.