I think a lot of this comes down to the fact that Irish experiences of the World Wars weren’t all the same. Plenty of Catholic Irish men did fight in the British Army, including in WW1 and WW2. But many did so in really difficult circumstances. Many signed up due to poverty, lack of work, and the promise that supporting the war would help deliver Home Rule. That promise was never delivered after the war, and many Irish families, mine included, felt betrayed by that.
You’re absolutely right that atrocities happened on all sides during the Troubles. But for lots of people in NI, the British Army were physically present, in their streets and homes, not from the past. I myself have first hand experience of brutal behaviour from British soldiers patrolling the streets and my parents know unarmed, innocent people who were killed by them. So while the poppy is intended as remembrance, in NI it is often conflated with very recent trauma, identity and politics in a way that isn’t the case elsewhere in the UK.
As for the Royal Irish Regiment, it doesn’t exist because some Irishmen “don’t want to stay at home”, it exists because the British military recruited heavily in Ireland. Partly due to strategic need and partly because joining the army was one of the few routes out of extreme poverty at the time. Some men also believed service would help secure freedom for Ireland from British rule. So again, the meaning of that service is complicated and not experienced the same way by everyone.
Anyway back to the poppy. None of this is to say people shouldn’t wear a poppy. Anyone who wants to honour the fallen should absolutely do so. It’s just that for many Irish people, remembrance takes a different shape, and that deserves the same respect.
And I think part of the difficulty is that this is British history. Northern Ireland is part of the UK, but a lot of what happened there isn’t taught or acknowledged widely in Britain. So people often don’t realise why the poppy can feel different to those of us who grew up during the Troubles and the history of Irish involvement in the world wars being very close with it being our grandfathers and great-grandfathers. It’s not about offending anyone. It’s just that for some of us, our experience of that history is much closer to home, and remembering looks different for us because of that.