The reason the IRA are quiet at the moment isn't because N.Ireland is in the EU , this myth that Tony Blair talked the IRA into giving up violence with his GFA is bizzare.
The IRA was a spent force by the 1990s , all their top operators were either dead or in jail, the security forces had the organisation riddled with informers, it was McGuiness who approached the government looking for a way out as he himself was in fear for his own life.
There is huge resistance for a return to violence among the Roman Catholic community as they know that it only causes a backlash from the Unionist community and disruption of their lives by constant anti terrorist operations by the security forces in their areas.