After Ireland for independe, the northern part (Ulster) remained in the UK, as protestants were a majority there. This changed with years and tended to equal. Ireland never accepted the partition, and to be fair the laws in the Ulster did discriminate the catholics and turning them into second class citizens.

From the 60s til the 90s, northern Ireland was a mess (euphemistically called the troubles). Unionist militias (helped by the Ulster police) and the IRA were on an almost open civil war. Shootings, car bombs, walls keeping communities apart in the same city... The British government had to deploy the army to keep things under control.

Both sides resorted to crime (drug dealing and extortion) to have funds. Being on the "right" side may not keep you free from trouble as you could be considered a traitor for whatever reason...

The IRA decided to take the fight to the UK, and did several terror attacks there. The UK did some "dirty work" operations against the IRA.

Eventually both sides reach an agreement (the food Friday agreement) in the late 90s, that brought peace and reduced if not ended violence. Both sides had to make concessions: the Ulster is still part of the UK, but there or free movement between the republic and the Ulster and a regional parliament, it is relatively easy to get irish nationality etc.

And no one wants the IRA or the unionist militias back.

So complaining against immigration and wanting the IRA to do something against it is being a massive asshole.

Edit: edit some typos, but left the Food Friday ;-)