Troops went in in 1969 (this time). You could go back to the 12th century when the Normans invaded (my ancestors) if you wanted to be pedantic - British military meddling in Ireland has gone on for centuries (partly from the occasionally legitimate fear that the island would be used by other enemies of Britain, which hasn't been the case since 1945 (one benefit of the EU, I suppose - the French are hardly likely to respond to spats over the CAP by sending troops to Dublin).
As for flattening the prison, I'd expect nothing less of a British commander who thinks his troops are in danger. It's what soldiers do. It does, however, rather jar with the 'things are getting better and it's all a media con' line being spun by the politicians - it's more like something you'd see in Sierra Leone than a fledgling democracy 'getting back on its feet'. Proper countries don't need heavily armed SAS lads in mufti running around trying to find out what the bad guys are doing.
? Heavily armed, are they? Those paperclips can give you a nasty nick when used in the right hands.