Tries by Wales flanker Martyn Williams and a pair from wing Chris Czekaj, with a stream of penalties from Ben Blair, was enough to help the hosts see off a struggling Irish side to reach the Anglo-Welsh semis at the Arms Park last night.
Despite having faced the might of New Zealand six days before, Wales quintet Martyn Williams, Rhys Thomas, Gethin Jenkins, Tom Shanklin and Mike Phillips were called into action to boost the Cardiff side for this Pool B clash.
In contrast, London Irish brought a slightly weakened side and even with several internationals in their pack they struggled at the line-out as Cardiff constantly stole set-piece possession.
Despite an early drop goal attempt from Irish fly-half Shane Geraghty, it was one-way traffic from the Blues in the opening exchanges, yet London Irish opened the scoring.
Against the run of play, their Welsh scrum half Richie Rees made a break 20 metres from his own line. His pass fed wing Justin Bishop who stepped inside the half-hearted tackle of Chris Czekaj to touch down; Geraghty landed the conversion from out wide.
The reliable boot of Blues' fullback Ben Blair kicked Cardiff into the lead with three penalties to make it 9-7 as the home side camped near the Irish line. The pressure finally told when Martyn Williams found a hole in the defence and dived over with Blair slotting over the conversion.
Czekaj made up for his earlier missed tackle by handing off Bishop and rounded fullback Delon Armitage before touching down his own chip ahead making it 21-7 at the break.
London Irish fought back after the break with a period of sustained pressure but the game degenerated into a dogfight with only a Blair penalty and a late Czekaj try for Cardiff to show for the second 40 minutes.
The Blues will be back at the Millennium Stadium on March 24th with their semi-final opponents set to be revealed at Sunday's draw.
Cardiff Blues: Williams, Czekaj (2) tries, Blair (2) con, Blair (4) pens
London Irish: Bishop try, Geraghty con