The Ospreys wing gathered and began to celebrate a try as he crossed the line, only for referee TimHayes to rule that Bishop's earlier loose pass had gone forward.
Leinster were given a route back into the game deep into first-half injury time when Nick De Luca blocked O'Driscoll as the Irish province attacked and referee TimHayes awarded a penalty try.
The penalty count against Bristol increased as the Scarlets pressure grew and eventually referee TimHayes lost his patience and Maori centre Neil Brew was the unlucky man to be sent to the bin.