Ben White sparked a Merrion collapse to inspire Phoenix to a second Irish Senior Cup success, 38 years after their first.
It had seemed unlikely as the Anglesea Road side had reached 176 for 3 chasing a target of 212.
However, White's 5 for 34 changed the momentum of the contest completely, as he bowled John Anderson for 39, ending a stand of 79 with top-scorer Sam Harbinson, whose 60 included 7 fours and 2 sixes.
The dismissal of overseas professional Peter Francis for 0, courtesy of a stunning catch running back at cover by Tyrone Kane saw the belief grow, and at 202 for 9, Phoenix were probably ahead for the first time in the contest.
Jamie Forbes boundary swung the tide back in favour of Merrion with two needed from two balls, but Devender Ranolia was the Phoenix hero, having a distraught Tom Stanton caught down the leg side as he moved across the stumps looking to sweep fine.
The scenes of joy at the end showed just how much victory meant to Phoenix and their skipper Adam Chester.
Head Coach Tyrone Kane played an uncharacteristically restrained knock, but his unbeaten 44 was crucial in getting his side to 211, as was a flamboyant 37 (8 fours) from Nic Pretorius.
With their riches of spin they knew a total of over 200 would give them a fighting chance, and boy did they fight.
Incidentally, Player of the Match White's figures were the third time that a bowler has taken five wickets in a final, all leg-spinners!
Mark Gillespie's 5 for 33 in Strabane's 1998 win over Ballymena is still the best, White is second, with North County's Richie Lawrence 5-38 in the 2008 Fingal derby win over The Hills third.
Spin to win...
The margin of victory, 1 run was the second time this has happened in an Irish Senior Cup decider, with North Down prevailing by just a single back in 1995 against Bready in another thriller at Comber.
Irish Cup Final: Phoenix beat Merrion by 1 run
Mardyke, Cork, 17 August.
Phoenix 211 (44.2 overs: Tyrone Kane 44*, Levon Shields 40, Nicolaas Pretorius 37, Jack Lalor 21; Peter Francis 3-37, Melvin Devaraj 2-61, Max Sorensen 2-41)
Merrion 210 (49.5 overs: Sam Harbinson 60, Swapnil Modgill 42, John Anderson 39; Ben White 5-34, Amish Sidhu 2-31, George Dockrell 2-40)