This comment has it completely backwards. Munster Irish is one of the only two native dialects, the other being Aran Islands Irish, that pronounces "mn" as it's written. Every other Goidelic dialect, whether it be Irish, Scottish or Manx, has shifted "mn" to "mr." It's likely the variety of Irish spoken in Limerick also had this shift, but this dialect is extinct and is not reflective of the Munster Irish still extant elsewhere.
Historically (reconstructed for Proto-Celtic) these were ben-ā, ben-ās. By the time of Old Irish they had become ben, mná. So the singular case+number suffix (-ā) was dropped over time, but not the plural one (-ās), and the first syllable of the plural was reduced until its vowel disappeared. Then the b nasalized since it was next to the n.
ben-ā > ben- > ben- > bean ben-ās > bn-ā > mn-á > mn-á
You're hearing na mrá. As explained in some of the other comments, in Ulster Irish and Connacht Irish, certain words that contain an n after another consonant, such as mná and cnoc, are typically pronounced with an "r" sound rather than an "n" sound.