Irish faced an uphill battle from the early stages as Chris Hala'ufia was sent off after only 10 minutes.
Matt Garvey was sin-binned in the opening minute of the second half, to leave London Irish with only 13 players for 10 minutes.
Despite their two-player advantage, Welsh were unable to extend their lead until the 56th minute, when Ross added his fifth penalty of the day.
There was late hope for Irish when Sonny Parker was shown a yellow card. However, they were unable to capitalise in a game which saw few good tryscoring opportunities for either side.