The red-and-white Hoops will replace demoted side Ballina in Division Three next season, after full-back Liam Delaney pocketed the try winner in the seventh minute of injury-time.
Delaney plundered 13 points, including two penalties and a conversion, for the Munster Junior champions as they became the province's 16th senior club.
Despite two wins over Monivea and Naas, Nenagh Ormond did not have it their own way against Old Boys.
Nenagh backrow Robbie Powell grabbed a quick reply for the home side, but the inspirational skipper was denied a second on 76 minutes - with the game poised at 17-13 in Rainey's favour - as he was called back for a Rainey penalty despite grounding over the line.
Luck was on Nenagh's side in injury-time as a Trevor Connolly intercept set up Delaney for an unconverted score and a breathless one-point win.
In Saturday's other Round Robin tie, Naas - who started the day four points behind Nenagh - were try-happy at Forenaughts running in eight, including a brace from Mark Waldron, for a 42-0 demolition of Connacht champions Monivea.