IAA?
They are the regulator in Eire. Give them a call I'm sure they will have the answer.
In the UK private flights off your own water is no problem.
Private flights off other peoples water is a problem.
Public Transport flights off water are no problem so long as you comply with Chapter 11 CAP168.
There's a guy flies an American reg Grumman off a Loch in Ireland. You would do well to speak to him
Good Luck
Wear a wet suit
Sir George Cayley