It was the first Big West loss for the Titans (6-5, 4-1 in the conference). Irvine is 6-4, 2-3.
Kirsch scored on a header into the center of the goal after UCI's John O'Brien headed a corner kick by Mike Roach. Fullerton goalkeeper Mike Ammann was expecting a shot from O'Brien but the ball caromed to Kirsch.
Irvine goalkeeper Chris Taylor, a red-shirt freshman, had nine saves in his second shutout of the season.
In Big West Conference water polo:
UC Irvine 9, Pacific 6--The Anteaters scored five consecutive goals to rally from a 4-3 third-quarter deficit at Heritage Park in Irvine. Steve Froning scored three goals for the Anteaters, who also defeated UC Riverside in a nonconference game Sunday. The two victories raised the fourth-ranked Anteaters' record to 10-2, 2-0 in conference.