"I thought that we did okay in the second half, but there's a long way to October (the Heineken Cup first round game).
"We're probably behind the likes of the Scarlets (in terms of preparation) but we have to prep ourselves next week for a tough game in Wales."
OZO man-of-the-match Isa Nacewa touched down with the opening score of the match after some smart interplay with Shane Horgan in the fourth minute.
But three Ryan Lamb penalties - after 10, 20 and 26 minutes - pushed the visitors into a 9-5 advantage.
There were some impressive cameos for the hosts in the first half with back rowers Stephen Keogh and Sean O'Brien showing real aggression alongside the industrious Shane Jennings.
Though Leinster threatened sporadically, it was the Exiles who showed greater potency in attack and they chalked upa Steffon Armitage try (34 minutes) and a penalty try, which Lamb converted into the Wesley end, to thank for their 21-5 half-time lead.
Leinster made a raft of substitutions in the second half as a number of players from Ireland's successful Churchill Cup campaign made their first starts of the season.
A Jonathan Sexton penalty on 58 minutes was just reward for a spell of Leinster pressure.
Fergus McFadden made a string of daring breaks, while replacement Kevin McLaughlin also impressed in the second half.
Nonetheless, a late Lamb penalty restored London Irish's 16-point advantage and though the result was disappointing, it was a good physical workout for Cheika's squad ahead of the start of the Magners League campaign next weekend.
LEINSTER: Niall Morris; Shane Horgan, Fergus McFadden, Shaun Berne, Isa Nacewa; Jonathan Sexton, Simon Keogh; Ronan McCormack, Bernard Jackman, Stan Wright, Leo Cullen (capt), Devin Toner, Sean O'Brien, Shane Jennings, Stephen Keogh.
Replacements used: Girvan Dempsey for Morris (23 mins), Paul O'Donohoe for Simon Keogh (31), Cian Healy for McCormack, John Fogarty for Jackman, Kevin McLaughlin for O'Brien (all half-time), Malcolm O'Kelly for Cullen (56), Mike Ross for Wright, David Moore for O'Donohoe (both 61), Billy Ngawini for Stephen Keogh (74), Stephen Keogh for Horgan (76). Not used: Jason Harris-Wright, Eoin Sheriff, Chris Keane, Brendan Macken.