You'll require flight plans (paper ones) for your flights between NI and the Republic.
You'll require a GAR form to Customs and to Special Branch for your flights between Scotland and Northern Ireland, and also for your flights between Northern Ireland and the Republic of Ireland.
In the republic you don't need any specific customs permissions to land once you're not smuggling anything. If you are, then you have to report to a customs station
You'll need a flight plan for all flights within Ireland that start in, end in, or pass through controlled airspace. If you can avoid controlled airspace from take off to touch down, then you don't need one.
If you land at an uncontrolled airport, or one which has been downgraded to AFIS, then make sure you close your flight plan. This can be done with the last ATC service (not AFIS) before landing, or by calling Shannon Low Level on +353-61-471233 after landing.
You can lodge flight plans over the phone with Shannon AIS on +353-61-703750. You must lodge them 1 hour before take off. Expect them to insist on the 1 hour.
Your IMC rating will not be valid in the Republic.
We have no VFR at night, and no IFR without an IR.
All airports in Ireland are PPR.
From the 3 August, 2006 the Dublin CTA has been extended. This results in new Class C airspace around Weston airport, and the R15 becomes Class C airspcae. You can log onto the Eurocontrol website (if you can manage it) and find the Visual Approach Chart for Dublin (EIDW). This will shown you the new airspace.
I hope that helps!
dp