You may not need a visa for a semester abroad in Ireland, but you will need an immigration card to stay in the country. One of the things I wish I was given more information about was the process of applying for the immigration card.
When you first arrive in Ireland a passport stamping man will ask for your necessary paperwork and leave a green stamp saying you have so in so days to get yourself down to the Garda building (Irish Police Station) and become a temporary immigrant. This time can range from somewhere of 3 weeks, a month or two. I personally, waited until the last day imaginable (which I don't recommend because it makes traveling out of the country a lot easier.)
So first thing is first when it comes to applying to become an immigrant. Get all of your important documents and passport and get yourself to the Garda. Don't know where the Garda building is? It's on Angeleasa Street across from the Court House.
One thing I cannot stress enough is the amount of time you will be spending in the Garda building. Dedicate an entire day to this venture. You will wait for hours in an almost non moving line and just when you think you're close to your window, they'll take the longest 2 hour lunch break you could ever imagine.
So make sure to bring your necessary paperwork, passport, and something to keep you entertained. I suggest an iPod, a book, or school work. Then the process won't seem as grueling as it actually is.