Both were in the starting XV, Cullen at full back and Mullins as centre partner to Gary Connolly and both were involved in the move leading up to the opening score of the game from Mark Melbourne, Cullen providing the scoring pass.
Munster went further ahead with a second try, this one from the impressive Ryan Hartigan and both scores were converted by Paul Burke.
Leinster responded with two converted tries and a penalty to edge in front for a lead they never subsequently lost.
Mullins departed the action in the 57th minute to be replaced by Anthon Pitout, the strong South African looking particularly sharp and Cullen left the action soon afterwards his place at full back going to Denis Hurley, a youngster who looks to be one for the future.
Mossy Lawlor replaced Burke before the final quarter and he converted his own try to leave Munster trailing 29-21 before quick hands from Hurley allowed Pitout the space for a final try that set up a tense finish.
Munster: C Cullen (D Hurley); B Touhy, M Mullins (A Pitout), G Connolly, P Devlin; P Burke (M Lawlor), M Prendergast (F Murphy); T Buckley, F Roche (D Varley), E McGovern; B Madigan, R Hartigan (F Cogan); M Melbourne, A Kavanagh J O'Sullivan.