Leo Cullen's side, who propped up their European group last year, are now nine points clear at the top of Pool 4 ahead of their final round trip to Castres Olympique next Friday.

They had two tries on the board before Frans Steyn's red card for a reckless 26th-minute tackle on his opposite number Jonathan Sexton - Montpellier's third sending-off in as many European games.

Adam Byrne's try just two minutes later, adding to earlier converted efforts from captain Isa Nacewa and man of the match Jack Conan, had Leinster leading 24-3 at half-time.

Montpellier, who won October's home fixture 22-16, could not live with the Irish province's high-tempo play and completely caved in as tireless flanker Conan completed his hat-trick, and Luke McGrath, Cian Healy and Garry Ringrose also crossed in an eight-try annihilation.

South African fly-half Steyn missed a long-range penalty attempt in the opening minutes before a forward pass foiled onrushing centre Joe Tomane following two solid early scrums from second-placed Montpellier.

However, the Leinster attack ignited approaching the quarter hour, full-back Nacewa popping up twice with his second run resulting in a try wide on the right. After Nic White's cover tackle had denied Leinster initially, the subsequent lineout ball saw Rory O'Loughlin break through midfield and then Sexton's scooped pass fed Nacewa for his sixth try of the tournament, converted by Sexton.

Deft distribution from Sexton and Conan soon sent Nacewa scampering up the opposite left wing, retrieving his own kick and showing great hands to send the supporting Conan over for a deserved seven-pointer.

Barely two minutes later, a brilliant burst forward by winger Byrne had Leinster hunting for another try and Steyn's attempted tackle in his 22, which saw his swinging arm connect forcefully with Sexton's head, led to referee JP Doyle producing his red card following input from assistant Wayne Barnes and TMO Sean Davey.

With Sexton briefly off the pitch, Nacewa turned the central penalty into three more points and just as overworked Montpellier tried to draw breath, Byrne beat Joffrey Michel to Luke McGrath's box-kick near the right touchline, linking with Conan whose excellent return pass, under pressure from two defenders, saw Adam Byrne touch down and replacement Ross Byrne convert.

Adam Byrne's wing colleague O'Loughlin did well to defend a kick through and deny Timoci Nagusa a try closing in on the interval, and Leinster's well-organised defence, particularly in the maul, thwarted the visitors soon after the restart.

Scrum-half White's terrific last-ditch tackle prevented Rob Kearney from scoring, the Ireland international having replaced a limping Nacewa, but Leinster's bonus point arrived in the 54th minute when the strong-carrying Conan barrelled past two defenders to go in under the posts. Sexton's conversion took him past the 500-point mark in European Cup rugby.

Prop Healy's impact off the bench swung the scrum battle in Leinster's favour and the hosts added serious gloss with four tries - three of them converted by Ross Byrne - between the 63rd and 74th minutes. Scrum-half McGrath started the scoring blitz with a snappy break from a maul 25 metres out.

Healy muscled over from close range soon after, following some nifty interplay starting with Ross Byrne's cross-field kick out to O'Loughlin. Then Pierre Spies' attempted chip out of his 22 resulted in turnover ball and Conan's third score of the night, and Ringrose anticipated fly-half Ross Byrne's offload out of a tackle and wriggled away from three defenders for a Brian O'Driscoll-esque closer.