England invaded Ireland a very long time ago during a time when the leader of the British was very belicose. After a time it was easier to hold onto Ireland than to let it go. Eventually England was ready to step out but they didn't trust the Irish and the Irish didn't trust them, rightly so on both counts. It's led to somewhat of a stand off. Ireland has been set up with a provisional government more than once I believe and every time something happened it wound up getting blamed on the Irish political parties so their provisional government was taken away again. I don't know the situation very well but this is what I have gathered from what I have read.
The term Terrorism itself is rather loose and the IRA, among others, definitely were terrorists (it's up to debate whether or not they are still). They made attacks against the British not only in Ireland but in England too, including attacks against civilians.