Leinster have lost three finals and a semi-final in their last four Champions Cup campaigns [Getty Images]Investec Champions CupLeinster (24) 45Tries: Larmour 2, Ringrose, Frawley, T O’Brien, Conan, J O’Brien Cons: Prendergast 2, Byrne 3Harlequins (14) 28Tries: Northmore, Cleaves, penalty try, Beard Cons: Evans 2, BensonLeinster finished strongly to hold off a spirited Harlequins and launch their latest quest for a fifth Champions Cup title with a bonus-point pool 3 win at Aviva Stadium. Jordan Larmour’s double and tries from Garry Ringrose and Ciaran Frawley secured Leinster’s bonus point before half-time in Dublin, but Luke Northmore and Cassius Cleaves scores gave Harlequins a glimmer of hope.Ten minutes after the restart, Harlequins narrowed the deficit to just three points with a penalty try as Jimmy O’Brien was yellow-carded for a deliberate knock-on. But after Quins lost Jarrod Evans to a yellow card, Tommy O’Brien and Jack Conan touched down for the hosts before New Zealand international Rieko Ioane marked his debut with a try assist for Jimmy O’Brien. Harlequins did not leave Dublin empty-handed, though, as Oscar Beard’s late breakaway score gave the Prem side a four-try bonus point.Bath v Munster and Scarlets v Bristol after wins for Leinster and La RochelleSaracens sweep aside Clermont for bonus-point winLeinster – who scored 10 tries in a 62-0 thrashing when the sides met in the last 16 at Croke Park in April – opened their account in the fifth minute when Larmour barged his way over in the corner.But Harlequins, who were without rested England internationals Marcus Smith, Fin Baxter and Chandler Cunningham-South, responded strongly as Northmore touched down under the posts after sustained pressure from the visitors.Quins, however, were unable to repel a relentless Leinster attack as Ringrose finished off a sharp move before Prendergast teed up Larmour’s second try with a superb crossfield kick. The visitors narrowed the deficit again when Cleaves sprinted through a gap in the Leinster defence to finish off a smart line-out move.But the hosts landed another blow before the break when Frawley pounced on the loose ball after Dan Sheehan’s overcooked line-out to snatch the bonus score.Having kept Leinster within reach, Quins heaped pressure on the four-time champions early in the second half when they were awarded a penalty try after a deliberate knock-on from full-back O’Brien with the visitors poised to score on the left wing.While O’Brien was yellow-carded, Harlequins were unable to score again before Evans was sin-binned, and their hopes of a statement away win were further dented when Tommy O’Brien produced a lovely piece of skill to knock a grubber kick past Cameron Anderson and score Leinster’s fifth.From there, the pressure told on the Quins defence as Conan finished a fine move before Ioane teed up Jimmy O’Brien to atone for his earlier indiscretion. While Beard’s score three minutes from time ensured a try bonus point for Quins, they have now lost nine in 11 games in all competitions this season.What’s next?Leinster travel to Leicester Tigers on Friday (20:00 GMT), while Harlequins host Bayonne on 14 December (13:00). ‘Ioane one of us already’ – reactionLeinster captain Caelan Doris told Premier Sports: “It’s a bonus-point win, but performance-wise we’re going to have to improve quite a bit going into Leicester away and as we get deeper into the tournament. “[There were a] few moving parts, not to make that an excuse, but we’re going to need to improve as we go on.”On Ioane: “He’s been great so far, he’s fitted in pretty quickly. It’s tricky coming in and learning a new language in terms of all the play-calling and whatnot, but he’s done a good job and he’s one of us already.”Harlequins head coach Jason Gilmore told Premier Sports: “[It was] a bit frustrating. I thought the boys put plenty of effort in there and a really proud performance for the fellas. I was really happy that we got the bonus point, I felt we deserved that. “I thought the endeavour was really good. I don’t want us to be happy with that though, we’re a good football team. Can we get 10% more football and show what we can do because scoring four tries with only 30% [possession and territory] I thought was a great effort.”Line-upsLeinster: J O’Brien; T O’Brien, Ringrose, Frawley, Larmour; Prendergast, Gibson-Park; P McCarthy, Sheehan, Clarkson, Snyman, J McCarthy, Conan, Van der Flier, Doris (capt).Replacements: Kelleher, Boyle, Furlong, Mangan, Deegan, McGrath, Byrne, Ioane.Sin-bin: J O’Brien (50 mins)Harlequins: Anderson; Cleaves, Beard, Northmore, Murley (capt); Evans, Porter; Wenger, Walker, Williams, Treadwell, Lewies, Carr, Evans, Lawday.Replacements: Turner, Hobson, Delgado, Launchbury, Schmid, Friday, Benson, Bradley.Sin-bin: Evans (59 mins)Match officialsReferee: Craig Evans (Wal)Assistant referees: Adam Jones (Wal) & Ben Breakspear (Wal)TMO: Keith David (Wal)Listen to the latest Ireland Rugby Social podcastListen to the latest Rugby Union Weekly podcast