What did the terrorists expect to happen?
The whole plot hinges on the fact that Sean Bean's brother was killed by Harrison Ford, and Ford gets stalked and hunted by these IRA terrorists because of that.
What were they expecting to happen in a mission like that though?? They were attempting to assassinate members of British royalty. There is a strong risk of danger and death in a mission like that. If it wasnt Ford that had killed his brother, it could have just as easily been police or bodyguards or military. I find it strange that Sean Bean's character took it so personally.