Scarlets maintained their hopes of reaching the LV= Cup semi-finals with a third successive victory that keeps them top of Pool Three.
Vili Iongi settled an absorbing match with his second try in the last minute.
Deacon Manu had earlier burrowed his way over for a score, with full-back Dan Newton converting all three tries and kicking two penalties.
Irish were always in touch thanks to Topsy Ojo's fine individual try and 14 points from Tom Homer's boot.
Scarlets: Dan Newton, Viliami Iongi Nort,; Gareth Maule (c), Adam Warren, Andy Fenby, Aled Thomas, Rhodri Williams; Iestyn Thomas, Emyr Phillips , Peter Edwards, Adam Powell, Dominic Day, Mat Gilbert, Johnathan Edwards, Kieran Murphy
Replacements: Kirby Myhill, Phil John, Deacon Manu, Craig Price, Dan Thomas, Liam Davies, Daniel Evans, Nick Reynolds
London Irish: Tom Homer; Topsy Ojo, Jonathan Joseph, Steven Shingler, Marland Yarde, Daniel Bowden, Darren Allinson; Clarke Dermody (c), Brian Blaney, Paulica Ion, Nick Kennedy, Bryn Evans, Kieran Low, David Sisi, Richard Thorpe
Replacements: Pete Austin, Max Lahiff, Faan Rautenbach, Bob Casey, Kieran Roche, Guy Armitage, Shontayne Hape, Ross Samson.
Referee: Nigel Owens (WRU).