Hey, I just wanted to contribute some knowledge for anyone on here who may need it...

Three Ireland offer an Unlimited Data 4G plan for only €20 a month. It includes

"All You Can Eat" Free Data 4G Access
Free Any Network Texts (Any carriers IN Ireland)
Unlimited Free Any Network Texts
Free "Three to Three" calls (so free IN network/carrier calls)
Free Any Network Weekend calls (Weekends commence at 12.01AM Saturday and end at 12.01AM Monday)

Somethings to note:

Adult content is blocked by default, so to get the filter turned off you need either contact customer support or when you are getting your SIM show them I.D. proving you are over 18 and tell them you want to make sure adult content isn't blocked.

Your €20 "Top Up" (we call them refills) is used when you call out of network lines.

You only need to "Top Up" €20 a month to get a month's worth of data, etc. (I have a smartphone that I got a SIM for once and never topped up again, still has internet on it.

In Ireland you never pay, to receive calls or SMS messages, only make them.

You can use your device as a mobile hotspot, as long as it is a BYOD device.

If you intend to use this SIM in a tablet or mobile hot spot DO NOT TELL the sales associate you will be doing so, otherwise they will sell you a data only SIM and you downloads will be capped and limited.

I lived in Ireland for 12 years, go back many times a year to visit friends and family, and have business interests there, that is how I know this... all other carriers are pretty much crap in terms of data, so avoid Vodafone, Meteor, etc.