I've done Haverfordwest-Waterford, and Kilkenny-Haverfordwest. On the way over we could only make 1200 ft due to low cloud, and like someone else mentioned, we lost London Info halfway over. Waterford were friendly, asked us to report some place we couldn't find on our charts, and when we told them, asked for another point, enquiring if we could find it like we were two strange foreigners with little sketch maps! We came back IFR, cloud all the way - my co-pilot had an IMC; I just did the radio and played with the VOR etc.
Talk to someone; you want to be in radio contact when you're flying over 40 miles of water. Wear a life jacket and immersion suit at the very least; carry a liferaft if you can. 80% of people survive ditchings; half of those die of hypothermia before they can be rescued!!!!
You can hire all of these things from SEMS very cheaply; I don't have the phone number to hand - do a search, or if you can't find it, ask me again and I'll look it up for you.