I recommend the old infantryman's solution - stick your mobile in a couple of ziplock freezer bags.
Also texts use a lot less power than a voice call, so your battery will last longer. If you get an Irish cell you will need to use the full international number eg +44 or +353 etc.
I also have a hazy memory of a helo pilot who ditched in the Irish Sea years ago and found the water REALLY cold and was only saved by his survival suit (which people laughed at him for wearing) -perhaps someone with a better memory could give you more details.
Regards
EG