In that opening quarter they were guilty of a plethora of unforced errors but cut down on these to fashion a marvellous try in the 30th minute that saw James Coughlan and Johne Murphy do extremely well to send Niall Ronan on his way to the tryline.
Priestland drew the sides level on the 50th minute but O'Gara extended the margin to six points after two more penalties before Stephen Jones kicked the final score in the 68th minute to give Munster a 17-14 victory in round three of the Heineken Cup in Parc Y Scarlets.
Munster: D Hurley; J Murphy, W Chambers, L Mafi, S Zebo; R O'Gara, C Murray; W du Preez, D Varley, BJ Botha; D Ryan, P O'Connell capt; P O'Mahony, N Ronan, J Coughlan. Replacements: D Fogarty, M Horan, J Hayes, D O'Callaghan, D Leamy, T O'Leary, I Keatley, D Barnes.
Scarlets: D Newton; L Williams; S Williams, J Davies, S Lamont; R Priestland, G Davies; I Thomas, M Rees capt, R Thomas; S Timani, D Welch; A Shingler, R McCusker, B Morgan. Replacements: K Owens, P John, D Manu, K Murphy, J Edwards, T Knoyle, S Jones, V Iongi.
Referee: R Poite.