Leinster was beaten by Saracens in the final in May, but the 2018 Champions Cup holder started its quest to regain the trophy with a bonus-point Pool 1 win at RDS Arena.
Ringrose was the star of the show, the Ireland centre crossing twice inside the first 14 minutes and completing his treble after the break.
Ronan Kelleher also touched down on his European debut before Johnny Sexton rounded off a slick move early in the second half, the Ireland five-eighth also adding six points with the boot.
Tries from Dean Budd, Epalahame Faiva and Luca Sperandio were in vain as Benetton were second best in the all-Pro 14 encounter.
Challenge Cup holder Clermont hammered Quins 53-21 in the Pool 2 clash at Stade Marcel Michelin.
Alivereti Raka and Peceli Yato grabbed doubles as the Top 14 side scored seven tries, blowing Paul Gustard's side away.
John Cooney made amends for an incredible missed penalty by slotting over the winning points from the tee to give Ulster a 17-16 success in the other Pool 2 match at Bath.
Exeter Chiefs is the early Pool 2 leader after a 31-12 bonus-point win at La Rochelle, while DTH Van Der Merwe scored Glasgow Warriors' only try in a 13-7 home triumph over Sale Sharks.
Munster opened its European campaign with a 32-13 Pool 4 defeat of the Ospreys on their travels, Keith Earls and Andrew Conway among four try scorers.