We could get into a lot of discussion here about Irish history and whether or not people who try get an occupational force out are terrorists or freedom fighters. Maybe we shouldn't.
Suffice it to say that the problems started when England took Ireland as a colony. Violence breeds violence.
"The coming of Cambro-Norman mercenaries under Richard de Clare, 2nd Earl of Pembroke, nicknamed Strongbow, in 1169 marked the beginning of more than 700 years of direct Norman and, later, English involvement in Ireland. The English crown did not begin asserting full control of the island until after the English Reformation, when questions over the loyalty of Irish vassals provided the initial impetus for a series of military campaigns between 1534 and 1691. This period was also marked by an English policy of plantation which led to the arrival of thousands of English and Scottish Protestant settlers. As the military and political defeat of Gaelic Ireland became more clear in the early seventeenth century, the role of religion as a new division in Ireland became more pronounced. From this period on, sectarian conflict became a recurrent theme in Irish history."
Wikipedia