After waiting nearly an hour for their opponents to arrive Wilson's Hospital raced into a 14-0 lead in the opening quarter at Navan.
John Paradise crossed for the first and captain Philip Condell added another. Portmarnock never gave up battling and a touchdown from Luca Manfredini left the half-time score 17-5.
In the second period Condell grabbed another try and Gordon Peavoy went over for the winners fourth near the end.
Scorers - Wilson's Hospital: P Condell 2 tries, J Paradise, G Peavoy try each. A Kanu 2 cons and pen. Portmarnock: L Manfredini, M Doyle try each.