I'm in Dublin right now and wanted to summarize my recent experience using a MiFi dongle on the O2 network for the last 6 days. My wife and I have smartphones and a netbook - and a rental car. So why not use Google Maps for our navigation here in Ireland? Well, I couldn't get a prepaid SIM to work in my phone (thanks Sprint) so I purchased a Huawei E5330 Mobile WiFi and a prepaid SIM card from Carphone warehouse at the Omniplex shopping center near the Dublin airport. €40 for the dongle and €20 for the data (30 days and 7.5 gb). It's worked everywhere from Dublin to Waterford to Kinsale to Dingle to Tralee to Galway to Cong to Dublin. The only places we lost service was inside castles/mansions (rock does that!) and in the Connemerra (sic?) park region (on a back road shortcut, not a M/N highway). We get 8-10 hours of use out of an overnight charge and can always plug it into the car phone charger we brought along (microUSB connection). Only issue we had was with trying to set it up...the O2 network wants to text a confirmation number to your mobile phone - but we don't have an Irish number and it won't work with an american phone number. What to do? Sweet talk the girls at Carphone warehouse into plugging their mobile number into the setup screen and giving you the PIN that O2 sends them. ;)
Lastly, set up a Google Voice number and install the Hangouts app on your smartphone. You can call "local" to and from the USA and Canada for free over the wifi (MiFi or hotel). You can also text anybody using the Hangouts app and constantly update your Facebook page. It's kinda like we never left the USA (data-wise). lol
Now...to try to sell the MiFi and data SIM (probably at least 4.5gb left to use before the card expires on Oct 28) before we fly out on October 6th!