Leinster needed a late Gordon D’Arcy score to tie up a 16-8 victory at the Sportsground in an intensely-fought battle with Connacht.
Fionn Carr crashed over for Connacht in the first half while kickers Darragh Leader and Ian Madigan missed two penalties apiece, though Jack Carty scored one for the hosts just before the break.
Madigan then replacement Jimmy Gopperth (2) pegged them back from the tee in the second period before D’Arcy secured the win for Leinster late on.
The loss means Connacht sink to RaboDirect PRO12’s bottom following Zebre’s historic triumph at home to Treviso earlier in the day.
And Leinster took their chance to reclaim second spot after Ospreys and Ulster wins on Friday had seen them slip to fourth.
It was Pat Lam’s men who shaded the opening stages, scrum-half Kieran Marmion making several purposeful pick and drives but Leinster ensured nothing came to fruition.
The first penalty attempt of the match came the way of the visitors however and from a central position, Madigan slipped the ball just wide of the right post.
But Leinster continued to test the hosts’ resolve – D’Arcy asking the most questions – though again defence staved off attack.
The game well and truly exploded into life midway through the half, Robbie Henshaw providing the ammunition as Carr gunned home to round off a dazzling break from Connacht territory.
Carty missed a tricky conversion before Leader, starting at full-back in place of the injured Gavin Duffy, failed to extend the home side’s advantage from the kicking tee twice in quick succession
On the half-hour mark Connacht conceded a kickable penalty of their own at the scrum but Madigan thumped the ball against the bar from halfway.
Finally though, the Leinster fly-half split the uprights with a routine effort after his side forced a Connacht error in a rolling maul.
An expertly-taken kick by Henshaw then pinned Matt O’Connor’s men back into their 22 and when a penalty was forced, 21-year-old Carty assumed penalty duties to ensure his side went into the break five points to the good.
The game remained finely poised early on in the second half as both sides put their bodies on the line to stem wave after wave of attacks.
Eventually Leinster, led by Rob Kearney, began to scramble Connacht lines yet a cheap penalty at the breakdown put paid to a promising period of play for the visitors.
Their first gilt-edged chances of the second half came when the home side were caught offside, leaving replacement Gopperth – who had replaced Madigan moments earlier – to chop down the deficit to two points.
The pendulum seemed to be swinging Leinster’s way when Jamie Heaslip picked up the ball at the base of a scrum and charged on, forcing a penalty for Gopperth to slot through once more.
However a late tackle from Heaslip gifted Connacht a chance to wrestle back the lead, only for Carty’s replacement Dan Parks to skew wide.
In the last ten minutes Connacht finally looked leggy under the constant menace of their Irish rivals – and after another well-worked maul, the impressive Gopperth offloaded to D’Arcy who bundled through.
Gopperth nailed the all-important conversion to leave the hosts needing more than a converted try and time petered out without a further score.