The big problem there is cost. It's about 12 km from the Mull of Kintyre to Northern Ireland. Dublin to North Wales is about 100 km. Either way, you have to build new infrastructure on both ends, and if you are serious enough to build a bridge this size, you could do lots of tunneling and backfill to make a competitive high-speed route from Belfast to Glasgow and Edinburgh, with then a train to London, and one could also run a line from Ulster to Dublin and do the full route. Yes, it's a roundabout route, but the cost of building the tunnel all the way from North Wales to Ireland would be far too much to stomach. Terrorism would be of little concern, as the overwhelming majority of the Irish Republicans have turned in their AR-15s (Long past fucking time on that front, too) and you just know that both Dublin and London would come down extremely hard on the perpetrators.