Peadar, a native of Dublin, did not learn to play the bodhran until his 40's when he became interested in Sean O Riada and Irish music. He was referred to Ó Ríada by an acquiantance who could not play for Ó Ríada for an event. He approached Ó Ríada and he handed him the bodhran with a simple rhythm: Butter and Eggs, Butter and Eggs, Butter and rashes, sausages, and eggs.
Peadar joined Ceoltóirí Cualann soon after the group was formed and joined the Chieftains in 1966 replacing David Fallon on bodhran. Mercier left the Chieftains in 1976, shortly after the group decided to go professional. His musical talents live on in his son, Mel Mercier