London Welsh edged out London Irish 15-9 at the Kassam Stadium after Chris Hala'ufia was sent off for the visitors early in the first-half on Saturday.
With try-scoring opportunities at a premium in the tight and tense battle of the exiles, it was left to the kickers to take advantage of any mistakes or ill-discipline and put points on the board.
And former Scotland fly-half Gordon Ross was on far better form than either of the men addressing the tee for the visitors, kicking four penalties in the first half to just one apiece from Tom Homer and Ian Humphreys.
Humphreys and Homer were both guilty of missing efforts at goal as well in the opening period but the crucial moment of the match came after just ten minutes when Chris Hala'ufia was given a red card for a nasty-looking tip tackle.
Irish even went down to 13 men at one stage after Matt Garvey was sent to the sin-bin but they still managed to create plenty of points-scoring chances in the second half despite their numerical disadvantage.
Unfortunately for them though, Humphreys and Homer were again found wanting with several more wayward penalty attempts and Ross made them pay for their profligacy by slotting over his fifth successful penalty of the day to make it 15-6 just before the hour mark.
Homer did finally hit the target with quarter of an hour remaining and the numbers were evened up for the final ten minutes after Sonny Parker was sin-binned but the hosts held on to record a vital win over their neighbours and rivals.
For London Welsh:
Pens: Ross 5
Yellow Card: Parker
For London Irish:Pens: Homer 2, Humphreys
Yellow Card: Garvey
Red Card: Hala'ufia
London Welsh: 15 Tom Arscott, 14 Dan Caprice, 13 Sonny Parker, 12 Hudson Tonga'uiha, 11 Nick Scott, 10 Gordon Ross, 9 Tyson Keats, 8 Adam Balding, 7 Ed Jackson, 6 Daniel Browne, 5 Kirill Kulemin, 4 Jonathan Mills (capt), 3 Paulica Ion, 2 Neil Briggs, 1 Franck Montanella.
Replacements: 16 Greg Bateman, 17 Tom Bristow, 18 James Tideswell, 19 Matt Corker, 20 Alfie To'oala, 21 Jamie Stephenson, 22 Alex Davies, 23 Seb Jewell.
London Irish: 15 Tom Homer, 14 Topsy Ojo, 13 Sailosi Tagicakibau, 12 Shane Geraghty, 11 Anthony Watson, 10 Ian Humphreys, 9 Darren Allinson, 8 Chris Hala'ufia, 7 Matt Garvey, 6 Declan Danaher (c), 5 Bryn Evans, 4 George Skivington, 3 Leo Halavatau, 2 Brian Blaney, 1 Max Lahiff.
Replacements 16 Gerard Ellis 17 Jerry Yanuyanutawa, 18 Halani Aulika, 19 Kieran Low, 20 Jebb Sinclair, 21 Jamie Gibson, 22 Steven Shingler, 23 Jack Moates.
Referee: JP Doyle
Assistant referees: Paul Dix, Andrew Watson
Timekeeper: Brian Abrahams
Assessor: Jerry Wallis