Munster gave evidence of their intent as early as the 3rd minute when Cusack's break was carried on by Danny Barnes who ended up knocking-on in the tackle after making serious in-roads into the Llanelli defence.
The visitors then enjoyed their period of territorial dominace that ended with a penalty for their out-half Steven Shingler.
It was a lead that lasted until the 19th minute when Cusack blasted over a long distance effort and then added the points to Sean Henry's try just three minutes later.
Llanelli lost Adam Shingler to the bin in the 21st minute and referee Dave Walker's 15th penalty after just 31 minutes saw Peter O'Mahony go to the bin just before Shigler returned.
Minutes later Danny Barnes left the field injured to be replaced by Ronan O'Mahony
Munster: Sean Scanlon; AN Other, Ivan Dineen, Troy Smith, Simon Zebo; Declan Cusack, Conor Murray; Dave Ryan, Sean Henry, Darragh Hurley; Dave Foley capt, Ian Nagle; Billy Holland, Tommy O'Donnell, Peter O'Mahony. Replacements: Mike Sherry, Stephen Archer, Dave O'Callaghan, Paddy Butler, Paul Rowley, Gareth Quinn McDonagh, Ronan O'Mahoney.
Llanelli: Dan Newton; Jonny Lewis, Nic Reynolds, Chris Keenan, Justin James; Steve Shingler, Gareth Davies; Matthew Monaghan, Rhys Lawrence, Aled Hopkins; Adam Powell capt, Adam Shingler; Matt Gilbert, Nic Cudd, Ben Morgan. Replacements: Simon Gardiner, Craig Hawkins, Allan Powell, Duane Eager, Aled Davies, Owen Williams, Liam Williams.