DFGW, which had handled Kerrygold's advertising on a project basis since 2004, did not participate in the pitch.
Media, which is handled by Rocket, was not included in the review.
WCRS will develop a new campaign to cover all Kerrygold products, including packet butter, Softer Butter and the reduced-fat Lighter Softer Butter range, which was introduced in June in response to rapid growth in the lighter spreadables sector.
The Kerrygold marketing controller, Alison Palmer, said: "WCRS's campaign stood out to us as a big idea with flexibility across all media. We're very excited to be working together."
The WCRS chief executive, Debbie Klein, added: "We clicked with the Kerrygold team from our first meeting, and we're looking forward to producing new work with it."
Kerrygold has been the butter brand of the Irish Dairy Board since 1961.
Earlier this year, the brand embarked on an ad campaign through DFGW, which focused on its provenance and Irish heritage.