Angel of Death by Jack Higgins

They call themselves "January 30", after the date of a British massacre in Belfast. They are allied with no one, killing American diplomats and KGB agents, Arabs and Israelis, IRA gunmen and Loyalist soldiers. But they are definitely the enemies of peace--and they are plotting an assassination that will shatter an uneasy truce that reigns in Ireland.

Former IRA enforcer Sean Dillon must hunt down January 30 before they kill again. Before they spark another war. Before Dillon himself falls prey to the ultimate assassin--the Angel of Death...