Here you go, all the info you'll ever need:
http://www.prepaidgsm.net/en/ireland.html
I personally use "3" Sim cards whenever possible. Their pricing is very reasonable (in your case, 30 euro cents/min) and they usually have unbelievable deals on data plans.
Here's what the offer in Ireland:
500MB/day: € 5.00/day
2GB/week: € 10.00/week
1GB/month: € 9.99/month
10GB/month: € 25.00/month