Hercus, stand-in for England No.10 Charlie Hodgson, landed two first-half penalties and a 40-metre drop-goal as Sale built up a 9-0 lead against the run of play and then clung on for victory.
But that was only achieved thanks to a nightmare kicking display by London Irish's usually dependable Mark Mapletoft.
Going into the match, Mapletoft had kicked 136 points this season but in the first half he squandered numerous opportunities to put the Exiles in control.
Mapletoft missed his first four penalty shots – two of them from relatively easy positions – and was also wide of the target with two drop-goal attempts.
He finally got it right to open his account when his fifth penalty shot, from in front of the posts, went over in the seventh minute of first-half injury time.
Three minutes into the second half Mapletoft hit the target with his second penalty from seven attempts but then he narrowly missed a long-range effort in the 71st minute which would have salvaged a draw for Irish.
But they had only themselves to blame after dominating the game territorially, launching attack after attack which all came to grief through a combination of puzzling errors and stout defensive play by Sale.
Irish were denied a try midway through the first half when winger Scott Staniforth was judged to have narrowly lost out to Sale's Oriol Ripol in the race for Mapletoft's probing kick ahead.
But they failed to take advantage when Sale were reduced to 14 men for 10 minutes after number eight Pete Anglesea was sin-binned for stamping shortly before half-time.
Winger Paul Sackey and former England star Mike Catt, in the centre, provided the main attacking threat for London Irish, who dominated in the lineout, but the home side failed to make their advantage tell.
Sale's only real try-scoring chance was squandered in the second half when wing-threequarter Chris Mayor juggled and then spilled the ball in the process of exploiting a gap in the Irish defence.
With Wasps losing to Newcastle, the victory takes Sale into second place in the table.
But London Irish's fifth-successive league defeat leaves them in danger of being dragged into the relegation dogfight.
For London Irish:Pens: Mapletoft 2
For Sale Sharks:Pens: Hercus 2Drop: Hercus