Some more info on data in Ireland:
If you want to be PAYG and will be mainly using data I'd recommend Three if the coverage is any good in your area (which it should be).
Three are the only network on PAYG who offer an internet add on (€9.99 for 1GB) while all the other networks are 99cent a day for 50MB, which would see you paying €30 a month if you used it every day.
Three also have 3 Like Home where if you roam in the UK on Three UK's network you get cheaper calls back to Ireland.
Also when you top up by €20 you get 200 free any network minutes and 300 any network anytime texts, and also your 333 webtexts too. See the links below for all the info regarding Three
3 Pay Price Plan
IGb Data add-on