Cardiff Blues dented London Irish's hopes of reaching the LV= Cup semi-final with victory at the Arms Park.
Gavin Evans scored the game's only try with Rhys Patchell adding a conversion and a penalty.
Full-back Tom Homer scored two penalties for the visitors on a damp evening at the Arms Park.
London Irish must rely on Worcester beating Saracens if they are to reach the semi-finals of the competition.
Cardiff Blues: Tovey, O. Williams, Evans, Hewitt, Robinson, Patchell, Walker, Trevett, R. Williams, Bourrust, Hill, Down, Paterson, Young, Copeland.
Replacements: Hobbs for Trevett (53), Dacey for R. Williams (69), Filise for Bourrust (53), Hamilton for Hill (6), Jones for Young (69).
Not Used: Watts-Jones, Sweeney, Czekaj.
London Irish: Homer, Ojo, Joseph, Armitage, Yarde, Geraghty, Phibbs, Lahiff, Mayhew, Aulika, Low, Evans, Garvey, Sinclair, Hala'ufia.
Replacements: Humphreys for Geraghty (56), Moates for Phibbs (56), Yanuyanutawa for Lahiff (69), McGregor for Mayhew (69), Halavatau for Aulika (60), Gray for Hala'ufia (60).
Not Used: Sandford, Shingler.
Ref: Ian Tempest (RFU).