For costs, you should call the mandatory handling organisation. Details are all in the AIP They can give you a firm quote. The prices are not excessive in my opinion, but since opinions seem to vary widely on this forum as to what makes a landing fee "reasonable", you should probably satisfy yourself before taking off!
As Blueskis says, there is a special-branch notification requirement. This is currently 12 hours prior to flight. Not too bad, but an impediment for those last minute jaunts. Sometimes, talking directly with SB can get you around those kind of restrictions, so if all else fails, chat to them.
The water crossing is a big one, but it sounds as though you have that covered. The Irish sea is cold at the best of times, so don't count on surviving in the water without a survival suit for long.
Otherwise, there is nothing special to know. It is an easy airport to fly in and out of, whether under VFR or IFR. The weather can change quite quickly, making a last-minute check before you go well worthwhile.
Have fun.