Geraghty’s heroics secured victory as Exeter were outscored four tries to two, with back-row forward Blair Cowan crossing the line twice and flanker Tom Guest and wing Alex Lewington also dotting down.
Exeter led at the break as Australian Dean Mumm powered over and later nudged ahead through a converted Dave Ewers try and the immaculate goal kicking of fly-half Gareth Steenson, before Geraghty intervened.
London Irish remain 11th, level on points with Newcastle, after inflicting Exeter’s fourth consecutive defeat and Smith cited the memory of October’s galling loss in the reverse fixture as motivation for his side turning the tables.
“Most sides would have played for a penalty and we made the correct decision but very few ball players would want to step up like that and it shows the class of the man,” said Smith.
“Losing down there was the turning point of our season. They celebrated hard, it didn’t go unnoticed and it hurt us, so there was plenty of motivation today.”
London Welsh assistant coach Gordon Ross has admitted his side stand little chance of overhauling the 20-point gap at the foot of the table after slipping to a demoralising 38-7 defeat at fellow Premiership strugglers Newcastle at Kingston Park.
Samoan wing Sinoti Sinoti, Rob Vickers, Mike Blair and Will Welch touched down in the first period and Ally Hogg and Tom Catterick in the second, as Nick Scott scored the visitors’ solitary try in their 13th successive league defeat.
“It’s a massive gap between us and Newcastle and London Irish now but we just have to keep improving,” said Ross. “We played quite well last week against Harlequins and were very competitive, but today was a backward step.”
Reigning champions Northampton remain top despite succumbing 20-7 and seeing their five-match winning run evaporate against Sale on Saturday, while code-switcher Sam Burgess scored his first union try as second-placed Bath beat Wasps 39-26.
England skipper Chris Robshaw made his return from injury and Nick Easter his 250th appearance in Harlequins’ 32-12 win over Leicester, as James Hook’s long-range penalty saw Saracens fall 24-23 at Gloucester.