Basically it would depend on the structure of the IRA and Sien Fein. There would no doubt be 'revenge' attacks and some internal manoeuvring. However would his replacement(s) be more or less fanatical than him. Its still a while before he starts turning his back on the Irish imperial dreams and starts talking about accepting peace and democracy. Would the modified leadership do that sooner or later? Would it be more efficient or less in either the terror campaign or any later attempt to change to a peaceful path?
Possibly the one lasting change might be if such an attack persuaded more Protestant groups to become more violent themselves. Such a 'success' might prompt emulation. In which case it could become a lot bloodier and we might well not have the partial peace of today.
Steve