To say the UK and Ireland have had explosive interactions in the past would be a bit of an understatement. There were spates of car bombings, shootings, and other forms of violence all along the border of Northern Ireland (Due to both sides believing Northern Island should be part of their nation).
Rowling probably takes a more pro-UK stance, and therefore is comparing the Irish people to the IRA (Irish Republican Army) bombers.
The conflict(s) were part of a period known as The Troubles (Wiki page linked)
Disclaimer - I’m an American outsider with an interest in learning about history, so I may not have the most accurate summary of events from either side of the story. The Wikipedia article should summarize incidents much better.
Wander@sh.itjust.works 1 hour ago
Americans seem to miss that the IRA (Irish Republican Army) were terrorists. Lots of people were killed including children.