A week after ending the 100 per cent record of leaders Saracens, the Warriors almost beat second-placed Irish.
They went 13-9 ahead when their scrum earned a 52nd-minute penalty try to cancel out former Newcastle winger John Rudd's first try since joining the Exiles.
Lamb struck in the 61st minute to level the scores.
Fly-half Willie Walker kicked 11 points for Worcester, but boss Mike Ruddock said: "We seem to have the ability to draw with top teams but I thought it was a winning performance from us."
Irish, who lost their previous home match to mid-table Newcastle, hope injured stars Delon Armitage, Sailosi Tagicakibau and Topsy Ojo will return before Christmas.
Coach Toby Booth said: "Credit to Worcester. They dominated the kicking game."
Danny Cipriani returns from injury today as Wasps take on Leicester.
The out-of-favour England fly-half broke his fibula against Northampton at the beginning of October but was yesterday named on the bench after a nine-week lay-off.
Wasps boss Tony Hanks said: "Danny was in good form before the injury and he is keen to pick up from where he left off."
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