It enabled them to experience the rarefied atmosphere of third place in the Allied Dunbar Premiership before a crowd of nearly 8,000 at the Stoop.
Almost as sweet for their coach Dick Best is that it came, of course, at the expense of his former club Harlequins. Best was shown the door at the Stoop two seasons ago but he didn't arrive by the tradesman's entrance yesterday.
The Irish, who are dangerous from almost anywhere, scored three exhilarating tries in a game in which the tackling was predictably blood-thirsty and in which Quins came off second best despite enjoying the lion's share of possession.
The Exiles (the name now refers more to players from the southern hemisphere rather than the Emerald Isle) suffered a blow when they lost their New Zealand scrum-half Kevin Putt just before the kick-off.
Putt strained a calf muscle in the warm-up and was replaced by Peter Richards (an Englishman). However, it was not as damaging as the loss of Zinzan Brooke to Quins in the ninth minute. The player-coach damaged his right knee and went off. To bring what they described as more composure to their game, Quins had relieved Thierry Lacroix of the captaincy and handed it to Brooke.
With Brooke's departure, Lacroix regained the responsibility, but it was short-lived. In the 44th minute the French stand-off was stretchered off with a head injury. Fortunately, the injuries were not serious enough to prevent the casualties from watching the remainder of the match.
Quins were also handicapped when their scrum-half Huw Harries limped off after 29 minutes.
Irish scored their first try through their captain, Conor O'Shea. It stemmed in midfield from a sublime pass from Stephen Bachop to Nick Burrows and the centre made huge inroads before finding O'Shea clear to his left. The conversion by Niall Woods on the stroke of half-time gave the Irish a 7-3 lead after John Schuster had kicked a penalty in the 31st minute.
Whatever satisfaction Burrows had derived from his significant role in O'Shea's try was obliterated in the 53rd minute when his intended pass to Woods was anticipated by Darren O'Leary who intercepted and ran 50 yards. Schuster's conversion put Quins 10-7 in front but four minutes later the Bachop-O'Shea partnership worked again.
The former New Zealand stand-off slipped a perfect inside pass to the full-back who found an angle at full speed to make the line despite the attention of a couple of tacklers.
Woods, who converted O'Shea's second try, added a penalty midway through the second half and the coup de grace was delivered in the 67th minute. When the Quins No 8 Chris Sheasby's pass from a fell behind Nick Walshe, Bachop hacked on and Woods picked it up before embarking on a glorious run which ended with him giving a try-scoring pass on the right wing to Justin Bishop.
Harlequins: J Williams; D O'Leary, P Mensah, J Schuster, D Luger; T Lacroix (D Officer, 44), H Harries (N Walshe, 29); J Leonard, K Wood, G Halpin (D Barnes, 62), G Morgan (B Davison, 75), G Llewellyn, Z Brooke (capt, R Jenkins, 9), C Sheasby, A Leach.
London Irish: C O'Shea (capt); J Bishop, N Burrows, B Venter, N Woods; S Bachop, P Richards; N Hatley, R Kirke, K Fullman (R Hardwick, 73), N Harvey, M O'Kelly, J Boer, R Gallacher, R Strudwick.
Referee: B Campsall (RFU).