Clontarf took a big step toward topping the league table on the back pitch at the Aviva on Saturday.
Not only did this deserved victory assure them of promotion but it also means that they are virtually certain of a place in the semi-finals.
Only a complete collapse in the final two games, and Buccaneers picking up close to maximum points from their last three outings, will deny Clontarf a play-off spot.
Clontarf, now 12 points clear at the top of the table, are quite capable of giving a good account of themselves against any of the Division 1A qualifiers.
Understandably, their Director of Rugby Bernard Jackman was reluctant to get ahead of himself but he did admit that the signs were good.
"It would be great to combine promotion with a place in the play-offs," said Jackman. "I believe that, with the quality of players we have, we should be a Division 1A side. And, assuming that we do make it into the semi-finals, we will look forward to the challenge.
"Obviously, we will be outsiders but Old Belvedere came out of Division 1B last season and very nearly beat St Mary's."
Clontarf are a much stronger outfit than the side which was relegated from Division 1A last season. They have brought in a number of experienced performers, and some of their younger players have come through.
It's a combination that has helped them to lose just two games in this campaign.
Importantly, they have developed the habit of winning tight games, something they repeatedly failed to do when relegated from Division 1A.
This was a match they could quite easily have lost. Down to 14 men following the sin-binning of Martin Garvey five minutes from the end of normal time and leading 20-16, they defended desperately.
But they held out and subsequently made certain of victory with a penalty from substitute Martin Dufficy.
From Lansdowne's point of view, it was an opportunity lost as they failed to hold on to the ball and made some poor decisions close to the line.
Lansdowne, playing with the wind, led 13-10 at half-time. Kieran Lewis scored a try, converted by Eoghan Hickey; Hickey landed two penalties; and Clontarf replied with a try from Eoin Cremen, converted by Noel Reid, who also added a penalty.
But Clontarf moved 20-13 clear in the second half, hooker Aaron Dundon crossing for a try that Reid converted, and Dufficy kicking a penalty.
A further Hickey penalty reduced the deficit to 20-16 but Lansdowne could not manage the all-important try. A suspected broken leg suffered by wing Matt Healy added to Lansdowne woes, but they remain in third spot and still in the running for promotion.
Lansdowne -- D Hewitt; C O'Boyle, K Lewis, G Stafford, M Healy (S Gahon 72); E Hickey, J Cooney; J O'Connell, T Sexton, C Murphy; A McCullen, W Earle; D Heffernen, T Coneely, R Boucher.
Clontarf -- D O'Shea; M McGrath, K Lett, E Ryan, M Keating; N Reid (M Dufficy 52), E Legendre; B Barclay, A Dundon, C Kavanagh (D Merry 80); B Gissing, S Crawford; E Cremen, B O'Mahony, M Garvey.
Ref -- J Carvill.