Marc Stcherbina had a prominent game at centre for the Blues
Blues: (21) 31
Tries: Williams, Czekaj (2)
Cons: Blair (2)
Pens: Blair (4)
London Irish: (7) 7
Cardiff Blues secured their place in the semi-finals of the EDF Energy Cup as Group B winners after a comfortable win over London Irish on Friday night.
The visitors shocked the home side with a 13th-minute try from winger Justin Bishop after a Richie Rees break.
Ben Blair replied with three penalties before the Blues took full control with tries from Martyn Williams and Chris Czekaj to lead 21-7 at the interval.
Czekaj wrapped up the victory with his second try a minute from time.
The Blues, who had already beaten Wasps and Saracens, were denied a fifth-minute try when referee Nigel Owens saw a forward pass from centre Marc Stcherbina to winger Mosese Luveitasau.
And the home side were then given an early scare when the visitors scored a breakaway try on 13 minutes.
Blues scrum-half Mike Phillips was penalised for not releasing the ball after he was tackled inside the Exiles half, and opposing scrum-half Rees sprinted away after taking a quick tap.
The former Ospreys man raced deep into the Blues half and then delivered a perfectly timed pass to winger Bishop, who cut inside Chris Czekaj to score.
Fly-half Shane Geraghty added the conversion, but Blair responded with three penalties for the Blues to put them ahead 9-7 after 25 minutes.
The first-half comeback was complete when the Blues, who were totally dominant in the forward battle, scored two tries in the final 10 minutes.
Flanker Williams struck first as he ghosted through the Irish defence from 10 yards after a period of sustained pressure.
And Czekaj made the most of a favourable bounce near the Exiles tryline to score after he kicked ahead down the left flank.
The Blues had a lucky escape early in the second half when Nicky Robinson's clearance kick was charged down, but the fly-half recovered to ground the ball over his own tryline under pressure from Rees.
A much improved defensive display by the visitors prevented further tries for the Blues, and the home fans had to wait until the 70th-minute for the next scoring action as Blair kicked his fourth penalty.
With the Exiles down to 14 men after a yellow card for centre Seilala Mapusua, Czekaj found himself in acres of space to score his second try after Robinson's perfect cross-field kick.
And Blair's touchline conversion was fitting way to end a thoroughly satisfying match for the Blues.
Blues coach Dai Young:
"We've had three big wins against English opposition, three big scalps, and it shows how much we're progressing.
"We haven't won anything, but we're in the last four.
"That's the sort of thing that's been beyond us in the last couple of seasons."
London Irish head of rugby Brian Smith
"I was a bit cheeky in that I made a lot of changes from our normal line-up and they didn't really work for us
"But despite that, I don't want to take anything away from the Blues. They are a quality team.
"To score that many tries in these conditions was a fine performance."
Blues: Blair; Luveitasau, J Robinson, Stcherbina, Czekaj; N. Robinson, Phillips; Jenkins, Rhys Thomas, Filise, D. Jones, Goode, S. Morgan, Williams, Rush (capt).
Replacements: Shanklin for Luveitasau (65), Davies for Goode (70), Macleod for J. Robinson (74), Yapp for Jenkins (76), Evans for Phillips (78), Lewis for Williams (78).
Not Used: Goodfield.
London Irish: D. Armitage; Bishop, Feaunati, Mapusua, Tagicakibau; Geraghty, Rees; Collins, Paice, Lea'aetoa, Hudson, Casey (capt), McCullen, S. Armitage, Leguizamon.
Replacements: Fitter for Collins (40), Kennedy for Hudson (44), Everitt for Geraghty (60), Murphy for Leguizamon (65), Hodgson for Rees (67), Horak for Bishop (79).
Not Used: Mackie.
Sin bin: Mapusua (69).
Referee: Nigel Owens (WRU)