Replacements: Aaron Ferris, David McGuigan, John Gunson, Conor Piper, Michael Cromie.
Replacements: Scott Flanagan, Thomas O'Reilly, Mark Fallon, Robbie Glynn. Steve Toal-Lennon.
Playing towards the clubhouse with the wind advantage, the visitors were first to settle and had the majority of play during the opening exchanges.
St. Mary's had an opportunity to take the lead in the secnd minute after 'Hinch were caught offside in the midfield, but Gavin Dunne's penalty attempt drifted wide of the mark.
'Hinch were starved of possession as the Dubliners kept control of the ball. A gap in the 'Hinch defence in the 10th minute created the opening the visitors wanted and an neat inside ball to out-half Cathal Marsh saw him touch down just to the right of the posts. Dunne added the extras.
This try seemed to stir Ballynahinch into action and they started to play with a greater tempo and purpose. Richie Lobb converted a penalty goal in 16th minute after St. Mary's strayed offside in front of their posts.
In the 23rd minute a great break from flanker David McGuigan cut the St. Mary's defensive line in two and he almost made the whitewash. The ball was flashed right by Aaron Cairns, and Harry McAleese - spotting the blitz defence - sent a neat grubber kick through which was collected by Rodger McBurney who ran in to score. Lobb converted for a 10-7 turnaround.
The visitors responded well to this setback and quickly took control of the remainder of the first half, Matthew D'Arcy and Marsh using the wind advantage to kick to the corners.
Dunne had a long range penalty miss in the 25th minute and five minutes later McAleese made a crucial turnover under the 'Hinch posts, allowing the hosts to clear the danger and take a three-point lead into the break.
St. Mary's continued to dominate the game from the restart and within minutes they were back in control, forcing Ballynahinch back into their 22.
In the 46th minute 'Hinch infringed on halfway, allowing Dunne to kick for the corner. From the lineout, the Dubliners secured possession and started to maul towards the line with second row Ciaran Ruddock crashing over for a try which Dunne converted.
Dunne landed his second penalty of the afternoon in 49th minute after the home side had strayed offside, taking St. Mary's 17-10 clear.
Ballynahinch just could not gain any decent field position - they ran across the field and lacked any penetration to create threats.
St. Mary's played the smarter rugby and as the final quarter approached Ballynahinch's frustration cost them dear, as winger Lobb was yellow carded for a ruck infringement.
It was a case of damage limitation at this stage as the County Down outfit fought to stay within the seven-point margin and in the end they were lucky to escape with a losing bonus point.
St. Mary's will be happy with their performance on the day with their pack showing some good form upfront.
Number 8 and skipper Kevin Sheahan never stopped tackling all afternoon and together with flanker Gareth Austen and try scorer Ruddock, they proved to be powerful assets for the victors.
The 'Hinch pack did well to compete and they did have the better of the scrum exchanges with former Ireland Under-20 players Kyle McCall and Chris Taylor working well in the tight.
The home back row were also effective, in particular McGuigan who has recently returned to action after a lengthy injury. Paul Pritchard and Neil Faloon worked tirelessly all afternoon.
The St. Mary's backs caused the main problems with Marsh at number 10 adding real pace to the visitors' attack and out wide Ian O'Neill always made ground with ball in hand.
Speaking afterwards, Ballynahinch head coach Derek Suffern said: "I am very disappointed, it is certainly a game that we had targeted and felt that we really could have got a result today.
"I suppose at half-time we turned around pretty confident, 10-7 up and thought that with the elements and with the wind we could play a bit of field position and put the squeeze on them and get a result, but unfortunately we didn't.
"At half-time we spoke about the importance of good field position, I suppose we just didn't get the ball in the right areas to get that field position, and when we did kick I don't think we were accurate enough, we kicked direct to them and they counter attacked well.
"From that point of view we have learn the lesson that we are either kicking the ball down the tramlines or we are kicking the ball out to try and put some pressure on them."
He added: "The game in hand has now a major focus for us. Garryowen did not get a win at UCD and only picking up a losing bonus point as we did today, so we are as we were at the start of the day.
"We have got Old Belvedere next weekend, they are riding high up at the top of the table fighting with Clontarf and I think they got a good win today, so it makes it more important for us."
Referee: Nigel Correll (IRFU)