Saints came into the game on the back of a five-game unbeaten streak and having won their previous three Aviva Premiership Rugby matches in Reading. But in what was comfortably their worst performance of the campaign to date Saints shipped four tries in the 20 minutes either side of half-time to give them their worst league result in 18 months.
Saints will still go into the Heineken Cup break sitting top of the table thanks to Harlequins' loss at Exeter, but that was small consolation for Mallinder.
"We were very disappointing today," he said to the media. "Physically and mentally we didn't front up. There was no intensity from us and no accuracy, especially in the first hour, and although we talked a lot in the week about what we could expect today we didn't put it into action.
"As a group we're very disappointed. The lads know they didn't perform and we know we'll have to work hard in training this week and improve massively in all areas for the Glasgow game."
Glimmers of light were few and far between for the Saints, notably the performance of Ken Pisi, who was outstanding throughout. Nevertheless in the opening stages Saints had looked sure of themselves, controlling possession and going through the phases. But for all their time on the ball Saints were lacking the verve which had characterised much of their attacking play in previous rounds.
Credit should also be paid to the London Irish defence, which pressed and harried and restricted the visitors' chances to a solitary Stephen Myler shot at goal from long range and a lineout in the home 22.
Scoring chances were few and far between for the Exiles, too, with Steven Shingler missing his first shot at goal and Mark Sorenson winning a lineout against the throw five metres out from his own line.
Irish, on the other hand, were choosing not to contest the Saints' throws in the air, preferring to set themselves to repel the subsequent rolling mauls. This worked well in the opening quarter, stopping the Saints in their tracks.
However it was when the Exiles chose to send a jumper up that they were at their most dangerous. Myler had kicked Saints into a dangerous position on the home 22, but Irish won the ball back for Jonathan Joseph to send a grubber upfield.
Marland Yarde hacked ahead, and although Vasily Artemyev appeared to be there to cover a cruel bounce took the ball away from the Russian and into the hands of Joseph, who dotted down just inside the dead ball line.
It was an unfortunate stroke of luck for the Saints but it showed just how dangerous the Exiles could be given half a chance.
The home confidence was certainly visibly lifted, and when Lee Dickson had two box kicks charged down in quick succession it set up a dangerous position deep in the visitors' 22.
An intelligent run from Yarde took the winger close, and there were several charges around the fringes that Saints did well to hold back. Shingler's three points were bad enough, but there was more to come before half-time with David Paice diving over from close range and Ian Humphreys landing a long-range drop goal.
By the time the whistle blew for half-time Saints had conceded 18 unanswered points and were looking shell-shocked. It could have been worse, had the Exiles made the most of a powerful Sailosi Tagicakibau run that sparked an attack which took them once again into the danger zone.
That time a penalty let Saints off the hook, but as the teams trotted towards the changing rooms it was already obvious that the visitors needed to regroup and then get onto the scoreboard quickly.
They did just that, Myler landing a penalty after a superb Pisi run had taken Saints deep into opposition territory. However the visitors would have been buoyed by the way that they had held out considerable pressure on their own line, and when Joseph broke to put Topsy Ojo over in the right corner all that early work had been undone.
Saints were now playing a desperate hand but nothing was coming off, a wayward pass from Noah Cato to Pisi nearly allowing Yarde in, and only a desperate tackle forced Bryn Evans into touch. But while the defence scrambled that time Saints could not stop Jamie Gibson scoring the bonus point try a minute into the final quarter.
Pisi did score a deserved try five minutes later, with Phil Dowson following the Samoan over the whitewash five minutes from time. Myler added both conversions, but the Exiles' lead was insurmountable and Saints headed home to lick their wounds.
6mins SAINTS Miss pen Myler
11mins IRISH Miss pen Shingler
22mins IRISH Try Joseph 5-0
33mins IRISH Pen Shingler 8-0
36mins IRISH Try Paice Con Shingler 15-0
39mins IRISH DG Humphreys 18-0
HALF-TIME IRISH 18 SAINTS 0
46mins SAINTS Pen Myler 18-3
48mins IRISH Try Ojo 23-3
53mins IRISH Pen Shingler 26-3
61mins IRISH Try Gibson Con Shingler 33-3
66mins SAINTS Try Pisi Con Myler 33-10
70mins IRISH Pen Shingler 36-10
75mins SAINTS Try Dowson Con Myler 36-17
78mins IRISH Pen Shingler 39-17
FULL-TIME IRISH 39 SAINTS 17