Scrum-half Rees scored two tries on his first start for Irish
London Irish: (19) 36
Tries: Rees 2, Horak, Leguizamon, Flutey
Cons: Everitt 4
Saracens: (12) 24
Tries: Ratuvou, Penalty
Pens: Jackson 4
Andy Farrell's full debut for Saracens ended in defeat as London Irish began their EDF Energy Cup campaign with a bonus-point home victory.
Scrum-half Richie Rees, full-back Michael Horak and number eight Juan Manuel Leguizamon all scored tries as the Exiles led 19-12 at half-time.
Glen Jackson kicked four penalties for Sarries but Rees' second try extended the lead on the resumption.
Rikki Flutey grabbed another before a Kameli Ratuvou try lifted Sarries.
They also claimed a penalty try to close the gap to nine points but their comeback stalled when prop Ben Broster was sin-binned with 15 minutes left.
Barry Everitt, who had already landed four conversions, kicked a penalty to see the Exiles home.
Farrell struggled to make any impact in his first senior start for Saracens, 19 months after his original signing.
With just two A team games and 15 minutes as a replacement under his belt, the former Great Britain captain was again used at blind-side flanker but never really got into the game.
He is due to link up with the elite England squad at their Loughborough University training camp next week.
London Irish coach Brian Smith
"We are happy to blow some cobwebs out. We scored five tries and played some good rugby.
"It is a step forward, that is the most important thing for us, so we are happy."
Saracens flanker Andy Farrell:
"I am disappointed to be involved in a performance like that. I feel gutted but I've got to take responsibility.
"We never kept hold of the ball, we never got past two phases. It is frustrating not to be able to play how you want to play.
"When you play (number) six you have to do your job before you can get into the game and unfortunately we didn't get past two phases today."
Saracens director of rugby Alan Gaffney:
"Andy is still a way off and he has got some learning to do but he will get better the more games he plays.
"He is so disappointed with what happened today. It wasn't just him by any stretch of the imagination.
"His desire is still to learn and he tried his heart out. I don't think it will be long before we see a dramatic change. It is part of the learning curve at present.
"I still think his best position is at six. Everyone is suggesting he plays
12 but the running lines Andy has at the present time - he has a trait where he runs across field - is probably the worst trait a 12 can have.
"Consequently, with some of the things we are trying to correct it wouldn't
be a positive as far as 12 is concerned.
"They (the RFU) originally acquired Andy as a 12 but Andy Robinson has
changed his mind and at the moment he is happy to go down the route of six."
London Irish: Horak; D Armitage, Tiesi, Flutey, Tagicakibau; Everitt, Rees; Hatley (capt), Russell, Skuse; Kennedy, Hudson; Roche, Magne, Leguizamon.
Replacements: Lea'aetoa, Coetzee, Murphy, McCullen, S Armitage, Geraghty, Bishop.
Saracens: Scarbrough; Haughton, Ratuvou, Johnston, Penney; Jackson, Rauluni; Yates, Byrne, Broster; Vyvyan, Ryder; Farrell, Russell, Gustard.
Replacements: Ongaro, Mercey, Chesney, Seymour, De Kock, Sorrell, De Vedia.
Referee: B Robertson (RFU)