I have been to Ireland several times but only once by air. Apart from having a European Firearms Pass you will need your friend in Ireland to get you a Deerstalking Permit and Certificate for a Registered Firearm. The application forms need to be submitted well in advance of your trip in order to get the licences issued.
You then have to be sure that your intended airline carrier will actually allow firearms to be carried, I seem to recall that we flew Air Cunnilingus but I am not sure.
I go by ferry from Holyhead these days especially since the stalker I shoot with in Co Wicklow told me some horror stories about mishandling of his foreign guests rifles at Dublin airport. If you go by ferry you can only go on the steel hulled ships (not the Fast Cat) and be sure to clear the fact that you are wishing to transport firearms AND ammunition with the carrier at the time of booking. And don't forget to put your ammo in 'manufacturers original packing', yes the ammo must be in manufacturers original cardboard boxes (even if you reload) other wise they will not let you board (as happened to one of my mates last year).
Nobody seems to bother at the Irish side, all the hassle is at the UK end.
Phil