The grandparent is referring to the Official IRA, which is commonly referred to simply as the IRA as it has had the greatest claim to the name since 1969. During the period the OIRA was active, The Republic of Ireland was (and remains) an independent Catholic country. Northern Ireland is part of the UK, but has a majority Protestant population who do not wish to become part of the Republic of Ireland. Attempting to incorporate Northern Ireland into The Republic of Ireland against the wishes of the majority of the population of Northern Ireland sounds like conquest to me.
During the period that the OIRA was active, they killed more people in the UK than Islamic terrorists have done in total. They did not cause Catholic churches to empty and their supporters toured America (particularly New York) raising funds to buy weapons. The US Government refused to block this. It only ended when, for some reason, around 2001 it became unfashionable to fund terrorists.