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The Lancers trailed for the first time all season when Luca Moscone scored early for the Irish.
Emerson Alves scored the tying goal on a penalty kick with a minute left in regulation after Ben Bondy was fouled in the box.
Miles Carrington scored the winner in the first of two extra periods.
Lancers goalkeeper Nolan DeTamble shut out the Irish the rest of the way to help Ursuline complete its season undefeated.
The Ursuline Lancers won the Stratford St. Michael senior boys volleyball tournament Saturday for their second title in two weeks.
They beat St. Michael, Toronto Upper Canada College and London Monseigneur-Bruyere in the playoffs after placing first in their pool with St. Catharines Eden and Bruyere.