Sheehan follows on from East Cork duo Paul Townend and Davy Russell, with two winners each, and Innishannon's Aidan Coleman, who saddled the winner in the first race of the week.
Simply The Betts gave Harry Whittington a first Cheltenham Festival winner when lifting the Brown Advisory & Merriebelle Stable Plate Handicap Chase.
After going close this week with Rouge Vif (third) in the Arkle and Saint Calvados (second) in the Ryanair, also for owners Kate and Andrew Brooks, Whittington finally graced the winner's enclosure with Gavin Sheehan in the plate.
Simply The Betts (100-30 favourite) stayed out of trouble in front, together with last year's winner Siruh Du Lac.
In a dramatic race, Royal Village and La Bague Au Roi fell out in the country, while Siruh Du Lac came down two out, bringing down Imperial Presence and hampering several others.
Simply The Betts surrendered the lead to Happy Diva at the last, but fought his way back to the lead to score by a length and a quarter. Mister Whitaker was a length and three-quarters away in third, with Oldgrangewood fourth.
Whittington said: "It's about everyone else, I'm thrilled for them. We've had five goes this week and been getting closer and now we've got there.
"It's been a great team effort, Laura Collett (eventer) has done lots of work with him over his jumping.
"I wouldn't know what we'd do next, but he's a proper horse. He's still improving, but he's had some hard races this season. Today he was a little bit behind the bridle, whereas in January he was hard on it the whole way.
"I'm just so proud of him, but I don't know what we'll do know, he's hard three hard races since Christmas. We'll let the dust settle."