Replacements: Pat Flood, Darren O'Reilly, Rhodri Jones, Philip Maher, Gearoid McDonald.
Replacements: Mark Griffin, Sean Seymour, Darren Mulcair, Jamie Gavin, Alex Wootton.
O'Sullivan has joined the Garryowen management set-up in an advisory role, as the Rory Brosnan-led outfit bid to avoid relegation to Division 1B.
With a gap of 12 points separating them and Ballynahinch, this may prove to be an impossible task, but there certainly were signs of improvement from the Light Blues.
Defending champions Lansdowne enjoyed a large amount of possession early on, but they were being turned over by their determined opponents on a number of occasions.
Despite playing on an artificial surface, the wet and windy conditions were nevertheless restricting both sets of players, with Lansdowne in particular finding it difficult to execute their usual slick patterns of play.
They did eventually break the deadlock, 21 minutes in, when former Cork Constitution star Scott Deasy split the posts with a 30-metre penalty from the left.
As the half wore on, Garryowen were struggling to make headway and with the Dubliners applying huge pressure in the 22 towards the end of the period, it was only a matter of time before they crossed the whitewash.
Garryowen managed to keep them at bay initially, but following good work by the Lansdowne scrum towards the right flank, referee Mark Connolly awarded the hosts a hard-earned penalty try.
Deasy added the extras but Garryowen responded when out-half Alan Gaughan found his range from a 35-metre penalty with the last kick of the half.
10-3 down at the interval, Garryowen raised hopes that a shock was on the cards when Gaughan registered his second successful penalty three minutes after the restart.
This brought about a strong period of dominance by Garryowen, and with Lansdowne being forced on the back foot for an extended spell, there was certainly an opportunity for them to take advantage.
Although they could not quite break through the hosts' defence, the visitors did cut the deficit to the bare minimum with another Gaughan penalty after 54 minutes.
Lansdowne eventually got a grip on proceedings though, and Deasy's well-measured 61st minute penalty eased any nerves that they may have been experiencing.
Although a bonus point try now looked beyond reach, Lansdowne continued to search for five-point opportunities nonetheless, and full-back Tom Kiersey was set to pounce six minutes before the end - only for play to be called back.
A spirited Garryowen were persistent in their approach up until the very end of the tie, but try as they might, they could not create enough clear-cut openings.
However, their four-point reversal did at least ensure that they came away with a losing bonus point, which does marginally increase their overall league total to three points.
Referee: Mark Connolly (IRFU)