Flight plan is required for any flight into, outof or transiting class c airspace and allso for crossing FIR boundary. Thankfully there isnt much class c except for around the main airports (dub,sha,crk,crn,slg,idl,con,ker,wtd) evrything else is class g upto fl75 and above that is class c again. Again as posted before we dont do all the different classes of airspace like the UK just classC and G so in effect you are either in controlled airpace or your not.
It will be interesting to here your opinions of galway and your experience of operating thru' there as they dont have the best reputation when it comes to GA
PS pooleys ireland flight guide is excellent and has all the info you would ever need for operating in ireland and is the one i would recommend.