Clontarf lock Simon Crawford made good use of his bulky frame to secure a try to make it 9-5 at the interval.
'Tarf, who compete a division above Lansdowne in the AIB League, looked to have clinched it when Niall O'Brien was given a free run to the line.
But full-back Ross McCarron and centre Lorcan Kavanagh appeared out of nowhere and managed to barge O'Brien into touch. Lansdowne held their nerve and clung on for a four-point win.
LANSDOWNE: R McCarron; J Meagher, S Gahan, L Kavanagh, C Oosthuizen; D Hewitt, M Healy; K Cleere, R Quinn, C Murphy, A Maher (capt), W Earle, R Liberman, V O'Dowd, A McCullen.
CLONTARF: P Howard; M Keating, D O'Shea (capt), B O'Donnell, N O'Brien; M Dufficy, S Treacy; J Wickham, A Dundon, N Treston, S Crawford, B Reilly, H Stride, B Focas, M Garvey.
Replacements used: C Keegan for O'Donnell (68 mins), B Wood for Focas (78).
Referee: David Keane (IRFU)