Conor McKeon was the hero for Lansdowne as his nerveless injury-time penalty decided Saturday's low-scoring Division 1A top-of-the-table clash, with Clontarf beaten 9-6.
The Ireland U-20 international kicked Mike Ruddock's charges to their third straight Ulster Bank League win - and their second of the month on the Aviva Stadium's main pitch. The result ended defending champions 'Tarf's winning start to the new campaign.
The hosts broke the deadlock in the fifth minute when out-half Scott Deasy slotted a penalty from 30 metres. Deasy's opposite number David Joyce levelled via a 15th-minute penalty and he expertly split the posts again to give the Bulls a slender 6-3 interval advantage.
The introduction of underage internationals Dylan Donnellan and McKeon helped to give Lansdowne fresh impetus during Brian Moylett's sin-binning, although McKeon missed his first shot at the posts.
The former Gonzaga College pupil was on target from a more central position in the 67th minute to square things up between the last two league title winners.
In a nerve-wracking finale, referee Andy Brace awarded two penalties to 'Tarf on the hosts' 22 but Joyce failed to convert either, hitting the woodwork with the second of them.
Lansdowne then built a platform to snatch a last-gasp victory, McKeon doing the honours with his final place-kick two minutes into injury-time.
Cork Constitution enjoyed their first league success of the campaign as tries from young backs Rob Jermyn and Tomás Quinlan saw them defeat Ballynahinch 23-16 at Temple Hill.
Leinster Academy speedster Adam Byrne helped himself to two tries in UCD's 32-12 bonus point defeat of Dolphin at Belfield.
St Mary's College won at the third attempt when edging out hosts Old Belvedere 14-10 in a tightly contested Dublin derby at Anglesea Road. A first-half try from winger Conor Toolan was bolstered by a penalty try and two Gavin Dunne conversions.
In-form Terenure College tore Young Munster apart in a 42-0 demolition job at their Lakelands Park fortress.
Terenure ran in six tries in all - including a third of the campaign for winger Shane Donovan - as they climbed to the top of the table, level on 13 points with Lansdowne but with a superior +60 scoring difference.
Division 1B leaders Galwegians have young replacement Ciaran Gaffney to thank for their dramatic late 13-10 win over UL Bohemians at Crowley Park. The Ireland U-20 international had the final say with a drop-goal in the 82nd minute.
Second-placed Ballymena continued on the right road with a 24-12 success at home to struggling Dublin University, with new full-back Sean Taylor, the former Preston Grasshoppers and England Counties player, running in a brace of tries.
Elsewhere, out-half Jamie Gavin's 13-point haul helped Garryowen beat Corinthians 23-18 at Dooradoyle, while Buccaneers' Alan Gaughan tallied up 17 points in their 27-17 defeat of Shannon.
Despite a hat-trick of tries from full-back Rory Campbell, Malone lost 20-19 to Belfast Harlequins in a hard-fought derby at Gibson Park. No 8 Sean Reidy touched down twice for Quins.
Star man: Adam Byrne (UCD)
The Leinster speedster was in terrific form as he ran in two tries in UCD's bonus-point win over Dolphin.
One to watch: Conor McKeon (Lansdowne)
The Ireland U-20 star's stock continues to rise as he held his nerve to kick Lansdowne to a crucial victory over Clontarf.
Number of the weekend: 11
Teams who still have a 100pc winning record across all four divisions.