The water now will be as warm as it gets. If you leave it till next year, there may be additional problems, depending on whether Northern Ireland and Scotland are both in the UK, both in the UK and EU, or other possibilities.
P.S. I've flown a Jodel DR1050 to Ireland 3 times. Prestwick to Dublin Weston with Donegal to Inverness return, and twice Inverness to Sligo and return. Not taking the shortest overwater track.