While I was in Italy, I purchased a stick and sim card for prepaid internet access through Alice Mobile/TIM. Does anyone know who a reliable company is for Ireland to purchase the sim card for my stick?
I'm afraid I have no personal experience with Irish cell carriers, but I do know that a data SIM from the U.K. will roam in Ireland for a reasonable rate. So if you pass through the U.K. on your way to Ireland, you can grab a prepaid SIM. I've used the U.K. branch of T-Mobile and it has worked for me. When roaming, you need to buy what T-Mobile calls a Euro booster. I am fairly confident that there are local cell companies in Ireland that will offer a decent rate for data usage. Your USB stick should work in Ireland assuming it is unlocked.
You might try a place like Carphone Warehouse in Ireland. They sell prepaid plans for multiple carriers. I've used them in the UK and they usually can advise what the various options are.
I used Vodaphone for Internet access while in Northern Ireland. Not sure if they are also in the Republic. I was happy with the cost and the service.