Noel Reid’s 70th-minute penalty gave Clontarf a narrow final day victory on the main pitch at the Aviva Stadium, but it was not enough to secure the Ulster Bank League title for the north Dubliners.
Young Munster struck late at Templeville Road to defeat Young Munster 23-19 and finish top of the table in Division 1A, just ahead of a Clontarf side they beat last weekend.
Lansdowne, with nothing to play for, started this game under a good deal of pressure as 'Tarf's power in the scrum gave them a decent platform.
But the hosts opened the scoring in the 11th minute when winger Matt Healy danced through in the left corner after the headquarters side sprung forward from a scrum.
Out-half Craig Ronaldson missed the conversion, although he did find his range when adding the extras to centre Sean Carey's try five minutes later. He was put through by early replacement Steve Collins.
Clontarf had an excellent chance to get up and running on the 20-minute mark when full-back Richie Lane lined up a close range penalty.
However, his effort came crashing down off an upright and to make matters worse for Andy Wood's men, Ronaldson slotted a superb drop goal six minutes later for a 15-0 lead.
Lane finally opened Clontarf's account with a close-in penalty on the half hour, but Lansdowne responded in emphatic fashion. The dynamic Ronaldson evaded a number of challenges to collect the hosts' third try of the afternoon.
Things were beginning to look ominous for the title-chasing visitors, but they did finish the first half with a well-worked try from winger Timmy McCoy. Lane's conversion brought the deficit down to 10 points at the interval – 20-10.
McCoy's seven-pointer gave Wood's charges some much-needed momentum heading into the second period, and they cut further into the Lansdowne lead just after the restart.
A penalty and a try from Lane put just two points between the sides with 47 minutes gone. Lane did have a change to level the sides, but his conversion attempt came up short.
'Tarf's quest to be crowned league champions for the first time looked to be back on track. Yet, Lansdowne and Ronaldson had other ideas, and the elusive number 10 showed quick thinking to get through for his second try after a quick tap penalty.
A simple conversion by Ronaldson put nine points between the sides, but again Clontarf refused to let their heads drop, and a 58th minute converted try by hooker Aaron Dundon, after a long maul towards the line, put the tie firmly back in the melting pot.
Noel Reid, a second half replacement for Lane, missed a long range penalty in the 66th minute. However, the Leinster-capped youngster was accurate from a shorter distance four minutes later, much to the delight of the vocal Clontarf faithful.
Reid's three-pointer got them over the finish line, and it looked for a while like it would be enough to make them the division's overall winners, but the late drama at Templeville Road meant that they had to settle for second place.
Referee: George Clancy (IRFU)