That extra point was picked up in the final moment of a disjointed game when Cardiff tried to break from deep to salvage a draw they certainly did not deserve. However, they spilled the ball and Leinster pounced to grab a fourth try.
Coach Matt O’Connor will be just delighted to have nabbed all five points having lost two of their opening three games but they will need a big step-up in performance for the visit of old rivals Munster to the Aviva Stadium next weekend.
Leinster got on top early on and seemed poised to brush aside a Cardiff side who, like themselves, had already lost twice this season, but Matt O’Connor’s men did not build on that early promise and rarely threatened.
A couple of penalties from the right by Ian Madigan reflected that early dominance.
Rhys Patchell landed two from three penalties from the left to tie the match at 6-6 after 29 minutes.
Cardiff lost inside centre Dafydd Hewitt after a clash of heads with his opposite number Ian Madigan after 34 minutes.
The game burst to life two minutes from the break. The initial opening came from a skipped pass out wide on the left from Madigan to Rob Kearney and, when the ball was worked across, Jimmy Gopperth timed his pop pass to Rhys Ruddock to perfection to allow the flanker to score.
Madigan landed the difficult conversion from the right but Leinster were pegged back again by a Patchell penalty.
Leinster took over after the restart and just buried Cardiff with a devastating step-up in tempo.
And Mick McGrath, given his opportunity after a string of displays for Clontarf, repaid that faith with a try six minutes in.
He was an interval replacement for Rob Kearney who had picked up a knock to a hand and while he spilled his first ball, McGrath cut a fine line to take a ball from centres Madigan and Gordon D’Arcy and power through for a try which Madigan converted to make it 20-9.
There should have been no way back for Cardiff when Leinster quickly added a third try through D’Arcy. Madigan’s conversion pushing Leinster 27-9 in front.
But then it all started to unravel for Leinster. Madigan spilled a grubber as he retreated over his own line and Cory Allen pounced for a gift try which Patchell converted after 64 minutes.
Less than two minutes Sam Warburton raced through and Patchell added the extras to reduce the gap to 27-23 with 14 minutes remaining.
Madigan relieved the pressure with a penalty and Leinster could breathe a sigh of relief as Luke McGrath made the most of a Cardiff error at the end to touch down.
Leinster scorers: Tries: R Ruddock, M McGrath, G D’Arcy, L McGrath; Cons: I Madigan (4); Pens: I Madigan (3)
Cardiff scorers: Tries: C Allen, S Warburton; Cons: R Patchell (2); Pens: R Patchell (3)
LEINSTER: R Kearney; F McFadden, G D’Arcy, I Madigan, D Fanning; J Gopperth, E Reddan; M Bent, S Cronin, M Moore; D Toner, K Douglas; R Ruddock, D Ryan, J Heaslip.
Replacements: M McGrath for Kearney (40), J Conan for Ruddock (54), M McCarthy for Douglas (58), L McGrath for Reddan (64), T Furlong for Moore (64), E Byrne for Bent (69), B Byrne for Cronin (78).
CARDIFF BLUES: R Patchell; A Cuthbert, C Allen, D Hewitt, G Watkins; G Davies, L Jones; G Jenkins, K Dacey, A Jones; J Hoeata, C Dicomidis; J Turnbull, S Warburton, J Navidi.
Replacements: A Thomas for Hewitt (35), M Rees for Dacey (51), D Fish for Watkins (51), S Hobbs for Jenkins (53), T Filise for A Jones (57), M Cook for Hoeata (57), M Vosawai for Navidi (71), T Knoyle for L Jones (74).
Referee: I Davies (WRU).