Having seen fellow Pool Two pace-setters Edinburgh win away to Racing Metro in Paris on Friday night, the Welsh side responded in the perfect manner at the Madejski Stadium.
Sam Warburton's second-half try complemented a fine Leigh Halfpenny kicking display as the Blues look to reach the knock-out stages for the first time since the 2008-09 season.
The result ended London Irish's slim hopes of progress from the pool and ruined head coach Toby Booth's ambitions of marking 10 years at the club with a victory.
The Exiles made four changes from the side which beat Sale Sharks in the Aviva Premiership for today's match, with Adam Thompstone, Shontayne Hape, Faan Rautenbach and David Sisi all drafted in.
Gavin Henson was given his first run-out at inside centre for the Blues after impressing at full-back and was one of seven changes for the trip to Reading.
Cardiff have lost just one of their last five European visits to England and opened the scoring in the sixth minute.
Halfpenny proved why he is considered one of the game's most lethal long-range kickers with a penalty effort from his own half, although the Exiles levelled immediately through Adrian Jarvis.
The visitors' support were in fine voice and looked likely to celebrate a 13th-minute try when Michael Paterson broke free, but his run from the halfway line was thwarted five metres shy of the line by Jonathan Spratt.
Irish were let off once again when Halfpenny uncharacteristically screwed wide a straightforward penalty from the left as the game became scrappy, with most of the rugby played around the halfway line.
The home side began to impress as half-time approached and were rewarded for their attacking intent when Cardiff conceded a penalty and Jarvis fired over four minutes before the break.
The score put Irish ahead for the first time and it was extended further as the fly-half struck another penalty.
However, quick thinking by opposite number Dan Parks put the pressure on the home side from the restart and careless play gave Halfpenny the chance to reduce the deficit, which he duly took from 35 yards.
Cardiff looked rejuvenated after the break and regained the lead within two minutes.
Despite what looked to be an earlier knock-on, referee Neil Paterson waved play on and Casey Laulala darted down the left before popping the ball for Warburton to cross the whitewash.
Halfpenny converted from wide on the left only for the Exiles to deservedly peg the visitors back 10 minutes later, with Delon Armitage firing home from just inside the Blues' half.
As the players grew tired the game became scrappy, with Halfpenny scoring penalties either side of a Jarvis effort.
The latter missed a penalty with eight minutes remaining as the Exiles tried to reduce the deficit and broke forward in numbers, although Halfpenny's last-gasp penalty wrapped up a Blues victory despite Bradley Davies getting sin-binned with seconds remaining.
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