A brace of tries from Owen Morgan and further scores from Josh Flye, Osian Davies and Gareth Way sealed a league and cup double for the Ironmen.
Rhiwbina got off to a flying start with tries Brett Chatwin and Sion Summers, but were out-muscled by the Division 1 East champions.
Louis Millar and Chatwin scored late tries to pull the Squirrels back to within three, but Merthyr survived a late scare to take home the Plate.
Rhiwbina stormed into the lead after a well-worked try from wing Chatwin. Outside-half Josh Davies executed a pinpoint cross-kick which found Chatwin, and the Squirrels flyer was too quick for the covering Merthyr defence.
The underdogs stretched their lead after 17 minutes with another score out-wide. The Rhiwbina backs spread the ball to the right flank and Chris Fido offloaded back inside for onrushing full-back Summers to crash over – Gareth McCarthy added the extras.
The Ironmen replied just moments later through full-back Fyle.
The Merthyr forwards tried to shove there way over before releasing the backs who pushed the ball to Fyle.
The full-back dummied his way through the scrambling Rhiwbina defence and touched down to pull his side back to within seven points.
Flanker Davies then found a gap in the opposition defence to complete Merthyr’s 12 point comeback.
The back-rower darted through a hole in Cardiff side’s defence to score from 10 metres out, fly-half Steve Lang added the extra two points to draw his team level on half-time.
Merthyr, who are a division above their Plate rivals, took the lead after 47 minutes.
The pressure finally told when Morgan squeezed in at the corner following Luke Harwood’s offload, taking the score to 17-12.
Morgan scored his second soon after, the lightning winger rounded his opposite man and dived over in the same corner.
Way then crashed over for Merthyr’s fifth try to seal the win.
Millar and Chatwin scored late tries to take their team to within three points, but they couldn’t get over the line for the winning score.
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