Just thought some of you might find this update useful if you are planning on visiting Ireland.
There is a new Data-only deal from Meteor in Ireland - this is probably the best deal from any network in Ireland, for visitors. €10 per month for "unlimited data". It is not actually unlimited - there is a "Fair usage policy" of 7.5GB - but this is still pretty good for the price. The SIM is free - so €10 is the full cost that you need pay for 7.5GB.
http://www.meteor.ie/unlimited-data/
You can still receive voice calls and texts, but there are no outbound voice minutes or texts included in this. You could top up by an additional €5 if you wanted voice/text. However, with Skype/Viber/VOIP/Whatsapp etc, you might not feel the need to make outgoing voice calls. There is also a really nice Android app in Ireland called Cabbage. It allows you to send texts from your phone using the free Webtext service on Meteor. This will give you 300 free texts per month, which will look exactly like the ones you send from your phone.
A couple of other nice features on Meteor - you get free WiFi at all eircom Hotspots in Ireland (Airports, many hotels, cafe's etc). Also, when you leave Ireland you get free incoming voice roaming right across Europe, and data roaming only costs 99c per day for 50MB - one of the better data roaming deals in Europe.
Meteor has an excellent network in Ireland - they were the first to launch 4G - however, the 4G is not yet available on these prepaid deals.