I read that "Soldier Spy" book by the former MI5 guy.
Most people probably know this, but the IRA never really wanted to bomb us. They wanted to intimidate us, and to pursue their cause by doing so. They didn't deliberately kill many civilian targets. When they had planted a bomb, they would send a coded message to the security services, so that the area could be cleared.
However, the security services could not, at that time, tell the public that it worked that way. If they had said "don't worry, they always tell us if there's a bomb", then the IRA would have lost the intimidation factor, and they would stop sending warnings.
If the IRA killed too many people, they would lose what they perceived to be the political moral highground. They wanted to frighten people, without killing too many.
This is in contrast with the current wave of Islamic terrorists, who want to kill as many people as they can, and they are not concerned that this has an adverse effect on the public's sympathy to their cause.