BALLYNAHINCH travelled to Limerick with a point to prove after three defeats to Young Munster last season.
Adam Craig bulked up his pack for the physical challenge with Will Hopes partnering Ronan McCusker at lock and Matthew Burke starting at loose head prop.
Reuben Crothers returned for his first appearance of the season in the back row and the backs remained unchanged with the electric pace of Aaron Sexton and George Pringle a constant threat.
The opening half hour was a physical affair with a Young Munster penalty the only difference between the sides.
The game then burst into life before half time when Mark Best scored for Ballynahinch with a typical hard line allowing him to finish from short range.
The home side replied almost immediately before a Pringle break and offload put Hopes over just before half-time to give his side the half-time advantage.
It was more of the same in the second half when Claytan Milligan extended the Ballynahinch lead from a well-worked line-out move.
Young Munster came roaring back with another try to close the gap to just four points.
The game looked to have moved decisively in favour of Ballynahinch on the hour when a yellow card reduced the home side to 14 players.
The hosts had other ideas and they produced their best rugby of the match to score ten unanswered points while a player down to take the lead entering the final ten minutes.
Ballynahinch piled on the pressure as they sought to avoid another narrow defeat in this fixture and when Young Munster received a second yellow card with just five minutes remaining, the stage was set for the visitors to steal the victory.
Repaying Young Munster for their late smash and grab raid at Ballymacarn Park last December, Milligan duly delivered from another line-out maul and Conor Rankin nailed the difficult conversion to take all five points home to County Down.
The victory shot Ballynahinch up to third in the 1A table and they will be keen to follow this performance with a victory over Garryowen at home next weekend. All support welcome.
Ballynahinch (1-21): Matthew Burke, Claytan Milligan, Kyle McCall, Will Hopes, Ronan McCusker, Reuben Crothers, Zack Mcall, Bradley Luney, Conor McKee, Paul Kerr, Ethan Graham, Mark Best, George Pringle, Aaron Sexton, Conor Rankin, Tom McAllistair, John Dickson Jnr, Declan Horrax, Peter Heasley, Chris Gibson, Matthew Booth.