The Eagle Has Landed, a novel by Jack Higgins about an Nazi attempt to assassinate Churchill, featured an IRA operative who rendered assistance to a German commando unit tasked with the mission. In reality, a significant number of Irish pols would probably have backed - with men and equipment - a Nazi invasion of Britain, if they weren’t afraid of British invasion and occupation. The rancor against Britain ran deep, perhaps because of the Irish Potato Famine. In fact without the famine, Ireland might still be part of Britain, which might have meant much more left-wing British policies, if today’s Ireland is any indication.
“The rancor against Britain ran deep, perhaps because of the Irish Potato Famine.”
It goes back farther than that, at least to Cromwell’s conquest in 1649 and the establishment of Ulster.
The famine and about about 500 other reasons.