Hi KF
Just done a similar trip, using Skydemon to do the planning, GAR and flightplans. All easy.
Have a look at routing S via Strumble VOR. and across via Slany. Its useful to use IFR reporting points on your flightplan for FIR boundary crossing. We were in contact with London INFO and they handed us over to Shannon with a squawk. At 3000 feet we had full radio coverage.
Once in Ireland, flying is much simpler, with much better integration of services. You need a flightplan to enter or leave any controlled airspace, but in practice it's fine. We got cleared through the Shannon overhead at 2000 feet, flying from Inishmor to Waterford on one squawk.
make sure it gets closed after you land
Not required in the UK, but must be done in Ireland. An ATC unit will do it automatically, but a small airfield might not.