It makes the most sense to consider the IRA and the UK as two separate groups. The IRA getting their demands met justifies terrorism for them; bad means, good ends. The UK getting peace justifies capitulating to the IRA for them; bad means, good ends. Why is capitulation bad means? The same reason that giving in to any bully is bad - you lose, the bully gets power, and it sets a precedent.

That said, the UK government certainly did a lot to piss off the Irish and it's not clearly the case that in the events leading up to the conflict all of the UK's actions were great and the Irish's were horrible.