Celtic Cup Final: Saturday, May 14Munster v Llanelli Scarlets, Lansdowne Road, 5.30pm (live BBC Two/Setanta Sports Ireland)
Lions fly-half O'Gara, who has not lined out in Munster red since injurying his knee against the Dragons in March, returns in place of Paul Burke at fly-half as the province attempt to end Gaffney's three-year term with some long-overdue silverware.
"It goes without saying that it's great to have Ronan back in the squad," exclaimed Australian Gaffney, who begins the trek home to his native land on Tuesday.
"He has made a full recovery and we did think about playing him last week against Leinster, but in the end deemed it prudent to give him that extra bit of time.
"Paul Burke has done a great job for us in Ronan's absence."
Former Ireland Under-21 Devlin comes in on the right wing meanwhile, as X-rays have shown that regular wide man John Kelly has suffered a double cheekbone fracture - an injury picked up late in last Sunday's 23-17 semi-final win over Leinster.
The 31-year-old Kelly is also unfortunately ruled out of contention for a place in Ireland's squad - announced on Monday - for the two-Test tour to Japan next month.
Better news comes with Lions prop John Hayes' recovery from the elbow strain he picked up against Leinster.
A victory over Gareth Jenkins' Llanelli, only Munster's second in five outings against the Welsh Region, would secure Gaffney's second title win of his three-year tenure, following on from 2003's Celtic League success.
Retiring duo Jim Williams and Jason Holland are unfortunately unavailable for a well-deserved swansong due to injury, but Munster will be boosted by the Scarlets' recent record in Ireland, and against the provinces.
Jenkins' men have only won once in their six trips across the Irish sea since the autumn of 2003, and that was against Connacht three months ago. The Scarlets, skippered by Ireland flanker Simon Easterby, lost all three of their final league games this season - with both Leinster (31-25) and Ulster (16-3) securing home wins against them.
With Munster on a five-game unbeaten run against Celtic opposition, Gaffney looks keyed in for a happy ending to his Irish stay.
Head-to-head since 2003: Munster 1, Scarlets 3Celtic form: Munster: WWWWW; Scarlets: LLLWWTop League/Cup scorers: Munster: Paul Burke 154 points / Christian Cullen 8 tries; Scarlets: Gareth Bowen 152 / Aisea Havili 8
Cup records: 03/04: Munster: Quarter-finalists (lost 14-18 v Glasgow (away))Scarlets: Quarter-finalists (lost 12-14 v Connacht (home))
(15) Shaun Payne(14) Paul Devlin(13) Mike Mullins
(16) Jerry Flannery(17) Gordon McIlwham(18) Trevor Hogan(19) Denis Leamy(20) Mike Prendergast(21) Paul Burke(22) James Storey
Scarlets: Barry Davies; Garan Evans, Matthew J Watkins, Tal Selley, Aisea Havili; Ceiron Thomas, Dwayne Peel; Phil John, Matthew Rees, John Davies, Vernon Cooper, Chris Wyatt, Simon Easterby (Capt), Gavin Thomas, Andy Powell.Replacements: David Maddocks, Aled Gravelle, Adam Jones, Gavin Quinnell, Mike Phillips, Gareth Bowen, Salesi Finau.
Referee: Joel Jutge (France)