I do the reverse crossing quite often. If you need to stop en route, use Haverfordwest - no doubt. They'll sort out the customs end for you if you ring them. Have an estimate for Slaney or whatever point you plan to cross the FIR at, and one for coasting in. You'll put transfered at the FIR to Shannon on 127.50 and given a new squawk. If Wx is bad in Ireland there are lots of strips to stop at near wexford, they should be all included in latest software update for a Garmin 296 etc. Probably 20nm from Galway you'll be transferred to them, and given a join usually straight in. Its not that busy.
If I were you and could use 320m I'd land in Claregalway. Its 1.4nm from EICM and has no fee's at all. 04/22 and nice dry field. Does me the finest.
Irish