The Exiles appeared to have blown the chance of replacing Northampton at the top of the table following an unlikely comeback from Sale.
But England Under-18 international Joseph, who started in place of the injured Sailosi Tagicakibau on the left wing, intercepted Chris Jones' pass deep into injury time to secure the bonus point. Sale had looked finished after Ryan Lamb, Joseph and George Stowers had run in tries to help Irish establish a 26-6 lead as the final quarter loomed.
But instead of probing for the bonus point needed to overhaul Northampton, the Exiles were forced to mount a desperate rearguard action.
Sale's fightback began with a 58th-minute try from Anitelea Tuilagi before Ben Cohen added a second shortly after.
With fly-half Nick MacLeod producing a faultless kicking display - he finished with 16 points - Irish were left glancing nervously at a scoreboard that read 26-26.
However, two late penalties from Lamb helped them regain control in the final five minutes before Joseph scored the crucial fourth try amid a frantic finish.
Leeds Carnegie 8-17 Wasps
Leeds Carnegie slumped to their seventh successive Aviva Premiership defeat as Wasps returned to winning ways at Headingley with a 17-8 triumph.
Scrum-half Joe Simpson scored a superb try five minutes before the break to help the visitors into a slender 7-3 interval lead following a poor opening 40 minutes.
It was a much-needed victory for Wasps following their dismal 37-10 loss at home to Northampton last weekend, but the result left Leeds bottom of the table and without even a losing bonus point.
Harlequins 6-6 Bath
Lewis Moody prepared for England's clash with New Zealand next Saturday by playing the full 80 minutes of Bath's 6-6 draw with Harlequins.