ya i will also confirm that you can go private site UK to private site Eire. I do it nearly every weekend.
Just do you Special Branch (GA report) to your local special police branch. Depends where you are going for who to contact in Ireland customs wise but if you just ring the customs in Shannon or Dublin, or give Weston airfield ops a call they will point you in the right direction.
Usually the best thing to do is keep a proof that you did fax or whatever some customs in Ireland. If they want to meet you they will, you just need to prove that you did try and contact someone over there. Have a good trip.
....and remember that thing about the flightplan to cross controlled airspace. It will break your heart otherwise!