I can only speak for Dublin and I'd guess other comments are also referring to the capital. Some points I would add:
On top of tech giants mentioned previously, more recently: Airbnb, Dropbox, Qualtrics (Announced friday)
Large number of high quality domestic companies. Intercom, Datahug, Logentries...
Massive amount of free networking events. Great sense of community and willingness to help among them.
Several high quality incubators. NDRC, Wayra
Great government support & funding. IDA & Enterprise Ireland
Europe's largest technology event is held here. This October will have ~10k attendees, 8k flying in from outside Ireland. Disclosure: I work here
There's an article about moving to Ireland in Hacker News top 10
A few links published in the last 7 days alone:
http://www.irishtimes.com/business/sectors/technology/tech-f... http://www.siliconrepublic.com/start-ups/item/34294-ndrc-rec... http://blog.websummit.net/2013/09/19/a-letter-to-irish-start... http://connormurphy.blogspot.ie/2013/09/a-letter-from-irish-...