Just fly from Rosslaire to Haverdfordwest, probably 30 min or less with the wind from the west. Make sure you have a reasonable ceiling and good visibility so you can see the other side when you're at 2000 ft.
Remember the engine doesn't know it's over water.
The only thing you need is a life-vest, now as another precaution I would consider an immersion suit, if it goes wrong a life-vest alone won't help you at all in the cold water of the Irish Channel.