The sharp finishing of left winger Cillian Barry was all that separated these two fine sides at Templeville Road.
The winners took the lead against the run of play in the 16th minute when a twisting, cross-field kick from centre Dan Casey was picked up on the hop by Barry for a try.
In a classic encounter the northsiders reduced the arrears in the 34th minute when Boucher kicked a penalty after centre Sean Gibbons sparked a counter-attack with a superb side-door delivery out of contact.
Casey could not quite match Boucher, pulling his shot at goal left from 35 metres for a push off the ball. However, he didn't miss from closer to home in the 38th minute.
Then, Barry got away on the outside from Casey's pass three minutes into injury-time for the clincher.
CBC Monkstown – T Kelly; D Porter (K Shaw 56), D Casey, P Rodgers, C Barry; J Blennerhassett, R Jordan; J Stafford, A Holmes, E Clarke, J McAteer (capt), A Byrne, R Doyle (C Madden 56), T O'Callaghan, W Fay.
Castleknock – G Balfe; N Eastmond (A Hobson 59), M Boucher, S Gibbons, R O'Meara; P Murtagh, J O'Neill (L Salmon 59); C Stinson, S Pardy (J Gibney 52), G Murray, J Watson (D Duggan 44), K Keogh (d treacy 50), C Birmingham, R Boueguerra, S Donnelly (capt).
Ref – T Townend (Leinster Branch).