As I understand it, the Irish problem largely went away with 9/11. Suddenly a lot of "well meaning" (?) Irish Americans sympathetic to terrorism discovered that
I'm assuming that the specific Irish problem you refer to is the IRA (and others) campaign to reunite Northern Ireland with the rest of Ireland. We do have other issues
9/11 had nothing to do with the IRA's cessation of hostilities: they realised a long time ago that violence wasn't going to achieve their objectives and decided to take the democratic route. I am in no way a fan of the IRA or Sinn Fein (their political wing), but I give them credit for realising that democracy and negotiation was the way to go.
UK border installations (watch towers, armoured checkpoints on roads) were being dismantled in the early 1990s. I remember crossing the border in 1987 or thereabouts and seeing bunkers and machine gun posts. 5 years later, no installations at all.
If you think the system is working, ask someone who's waiting for a prompt.