When I took a course on terrorism, the course taught that he was the father of modern terrorism. Michael Collins’ methods of warfare were copied by the Algerians, then the PLO etc. The instructor claimed there were IRA members who trained the Algerians, and aided the PLO (in this case probably the Marxist Provisional faction of the IRA).
One more comment about the development of modern terrorism.
Collins only made war on British government institutions, not on women and children. It took the Algerians and the PLO to add that wrinkle to the method.