Operation armegedon is a possibility maybe
Operation Armageddon was never a real possibility. The men who planned the operation knew themselves that they had no capability to fight Britain conventionally - Ireland had half a dozen old jets, some World War II surplus howitzers, a couple of armoured vehicles and were severely outmatched in manpower. Britain meanwhile had three thousand troops in Northern Ireland when the plan was put forward, backed by APCs and there were RAF Phantoms and Harriers close enough to deal with any Irish thrust at Derry or Newry. No sane Taoiseach would have allowed such an operation to go forward, since it would only result in the gutting of the Irish armed forces, the jeopardising of Ireland's application to the EEC and the
worsening of the situation in the north
As for the OP, I suppose it is possible, but it would be difficult. Despite what some people on the American side of the pond occasionally seem to think the IRA never had the support or resources to turn Northern Ireland into something like Vietnam. Arachnid does make a good point though that Prime Ministers Powell or Benn could have made things a lot worse: Powell by cracking down harder on the IRA (in the process allowing the UVF and all those other lovely chaps to become a great deal worse), Benn by giving the Republic of Ireland (a state far less prepared to handle an insurgency like the one Britain faced) an even
worse insurgency
On religious cleansing, one could argue that that did occur to some extent on a very local level. Take for instance my neck of the woods, Newry. Before the Troubles one person in five in Newry was Protestant, and Newry's protestant population was one of Ireland's oldest (with St. Patrick's Church in the town dating back to 1578). Nowadays (depending on who you ask) that population has fallen to something between 9% and 5%. People were forced to flee their homes due to sectarian violence all over the place - Belfast and Derry in particular. But, thankfully, it didn't happen on the scale seen on occasion elsewhere