Replacements: Peter Weir, Jordan Gratton, John Gunson, Andrew Harper, Stuart Orr.
Replacements: Paul Neville, Neil Griseto, James Rael, Steve McMahon, James Frawley.
Ballynahinch played towards the clubhouse end in the first half, but it was the visitors from Limerick who made the better start.
A scrum infringement gifted Garryowen a penalty which out-half Jamie Gavin converted, sending his side three points clear after just three minutes.
'Hinch responded well and in the fifth minute a great break by full-back David McIlwaine almost put Ross Adair in at the corner, but the final pass was spilled with the line just metres away.
McIlwaine had 'Hinch level two minutes later with a well-struck penalty after the visitors were caught offside at a ruck.
The game was played at pace but lacked accuracy at times which added to the tense atmosphere. As infringements began to mount, Gavin and McIlwaine swapped two further penalties, in the 24th and 36th minutes, to make it 6-6.
Entering injury-time at the end of the first half, a great break from prop Kyle McCall got 'Hinch on the front foot for the first time.
The County Down outfit kept control and held their patience, taking play through a number of phases until the ball was moved wide to find winger Ross Adair at pace, and he dived over for an unconverted try, leaving the home side with a narrow 11-6 half-time lead.
'Hinch made a slow start to the second half and found it difficult to get any meaningful territory. Paul Pritchard was yellow carded for not releasing a tackled player in the 50th minute.
Despite going down to 14 men, the home side lifted their game, taking possession of the ball and controlling play.
John Gunson had been drafted onto the replacements bench as cover, so when Rob Greer injured his leg and headed to the dugout in the 60th minute the famous green headgear was donned once more and the years came rolling back.
Gunson wasted no time in getting into the thick of the action as 'Hinch pressed deep into the visitors' half.
In the 64th minute Gunson was in control at the back of a rolling maul, which rumbled into the Garryowen 22 until flanker Paul Neville collapsed it, seeing him spend the next 10 minutes in the sin-bin.
'Hinch clinched their precious victory in the 74th minute following a period of sustained pressure in the visitors' 22.
The 'Hinch pack initiated series of pick and drives close to the Garryowen line and then the backs released play across the 22 where Aaron Ferris stepped inside the tackle of James Rael and sprinted clear to touch down beside the posts. McIlwaine added the extras to make it 18-6.
With just over five minutes remaining, Garryowen pressed hard for a losing bonus point. It was a powerful defensive display from 'Hinch though and they eventually turned the ball over and cleared the danger to seal the four league points.
Speaking afterwards, 'Hinch head coach Derek Suffern said: "I am absolutely ecstatic after this win, our boys showed superb passion, work-rate and desire to defend in the game - that was the most pleasing thing for me, and it has improved massively from previous weeks. I think that was the big difference.
"It is funny, in a game like that, it is a bottom of the table clash, the margins are small, and we missed a few opportunities, gilt-edged opportunities in the first half.
"I think it would have put a total different feel to the game, and the momentum would have swung our way. We didn't take those chances and I suppose we showed good patience and good control to make sure that didn't affect us mentally.
"It was hugely important not to give them anything at the end. We put ourselves under a wee bit more pressure than we wanted at the finish.
"Again we showed great character, and a great desire to defend at the end. We gave them nothing and it was hugely important that we made sure that Garryowen stay well below us at this stage. From here on in we are looking up and that is a performance that we can build upon."
Referee: Nigel Correll (IRFU)