The Fighting Irish had a chance to tie, but missed a jumper at the buzzer.
The game was tight throughout and Avon trailed by four after three quarters before quickly assuming the lead in the final period.
Tim McCulley led the Irish with 22 points while Tommy Prospero added 15.
Avon will face (5)Mynderse in Saturday's championship game at Blue Cross Arena. Tip time is 3pm.
Mynderse got by (3)Addison 51-37 at Penn Yan HS Monday night.