|London Irish (19) 32|
|Tries: Cowan, Lewington 3|
|Cons: Geraghty 3 Pens: Geraghty 2|
|Saracens (23) 36|
|Tries: Streather, Wigglesworth, B Vunipola|
|Cons: Farrell 2, Hodgson Pens: Farrell 3, Goode 2|
A last-gasp try by Billy Vunipola meant Alex Lewington's hat-trick was in vain as Saracens pipped London Irish.
Tim Streather and Richard Wigglesworth scored for Sarries either side of a Blair Cowan try before Alex Lewington notched a brace.
Lewington completed his hat-trick to overturn Saracens' half-time lead, but two Alex Goode penalties and one from Shane Geraghty set up a tight finale.
The visitors broke Irish hearts at the death when Vunipola was driven over.
Saracens had come into the game full of confidence having thrashed Harlequins at The Stoop last time out and, when they crossed after just 50 seconds against the Exiles, they may have mistakenly thought they were in for an easy afternoon.
With less than a minute on the clock, a charge by Vunipola was followed by neat play from Wigglesworth and Goode to send Streather crashing over, with Farrell converting the try and then adding a penalty immediately after.
Irish hit back through Geraghty's superbly-timed pass which put flanker Cowan over but, in a ding-dong affair, Wigglesworth then spied a gap before running 25 metres to give Farrell a simple conversion.
The hosts returned fire with Lewington latching onto Chris Noakes' cross-field kick and the winger skipped past a couple of challenges to touch down the first of his three tries.
Geraghty added the extras but failed moments later when Lewington doubled his tally, racing onto Scott Steele's grubber.
Farrell had the last word of the half with a penalty as Saracens led 23-19 at the break, but it was the Lewington-Steele combination that teamed up again for the winger to complete his treble.
Alex Goode took over Sarries' kicking duties and his two penalties levelled things up at 29-29, but Geraghty restored the home side's advantage with his second penalty before he and Goode took it in turns with four missed shots at goal.
With just two minutes remaining, the visitors set up a rolling maul which rumbled towards London Irish's line and Vunipola was credited with the try that kept up Saracens' 100% start to the season.
London Irish director of rugby Brian Smith:
"We got our tactics right and we showed lots of guts and determination.
"We are relatively disappointed as we left three points out there and came out on the wrong side of the scoreboard.
"In the first half-hour we looked to be on the wrong end of a thumping and not many teams come back from that against a side as strong as the Saracens.
"We did so, and it was unfortunate that we didn't get the result at the end, but we'll take huge positives from the game."
Saracens director of rugby Mark McCall:
"I probably would have kicked for goal in the last minute, but we leave it to the players on the field.
"Our skipper Brad Barritt made a brave call to go for the attacking line-out and we executed it brilliantly for Billy Vunipola's try.
"Fair play to them - they came with a clever plan to kick in behind our defence but we should have dealt with it better.
"They are a young, energetic and physical side and a lot of teams will come here and not win this season."
London Irish: Fenby; Lewington, Mulchrone, Geraghty, Short; Noakes, Steele; Court, Paice, Aulika; Skivington (capt), Sinclair; Cowan, Treviranus, Narraway.
Replacements: Ellis, Parr, Halavatau, Leo, Low, Guest, O'Leary, Sheridan.
Saracens: Goode; Ashton, Streather, Barritt (capt), Wyles; Farrell, Wigglesworth; Gill, George, Longbottom, Hamilton, Smith, Joubert, Burger, Vunipola.
Replacements: Spurling, Barrington, Johnston, Botha, Wray, De Kock, Hodgson, Taylor.