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
I find so confusing and misleading that the spelling is non accompanied by the pronunciation (as here, «na mná» pronounced «na mrá»). This divergence of spoken language from spelling (or spelling from spoken language) looks more like an English cultural issue (with unpredictable pronunciation, where «ghoti» could be pronounced «fish») that I expected non to affect the beautiful Irish writing. I understand that different dialects have different pronunciations, but in the dialects of other languages I know (Italian, Spanish, German...) the spelling is changed accordingly to match the pronunciation (or, at least, the pronuncuation is not considered correct and wouldn't be used in a course - for example, no one would use Andalusian pronunciation as an acceptable/standard in a Spanish course).
I have no authority to decide in which aspects Irish should use English habits, but expressing this uncomfortable feeling I had when I started learning English makes it a bit easier to deal with it. Am I the only one who feels so?
The status of the dialects in Irish is not the same as those dialects of Italian, Spanish or German. Irish doesn't have a "standard" spoken dialect, the standard is a written standard only, and while there are certain words that are associated with specific dialects, and have their own spellings, the particular pronunciation shift in this word is well recognized and standard, and words like mná, cnoc and cnámh have a standard spelling, but are pronounced with an "r" sound in certain dialects. Both this pronunciation and the pronunciation with "n" are considered correct.
In that case, why aren't both forms, «mná» and «mrá» accepted as written forms? In Spanish you can write «aquí» or «acá», it's not like only one is written, but both are spoken. Spelling and pronunciation work in parallel. When it's accepted that they don't match, we are on the slippery slope that English (or Danish, and in part French) entered long ago. One of the many things I like of Irish is that when I see a new word I already know how to pronounce it, I don't need a key (until now).
If you want to study Spanish, go ahead and study Spanish, with a word having multiple different spellings. The accepted spelling of mná in Irish is mná, even among those who pronounce it, and words like cnoc and cnámh, with an "r" sound. You don't have to pronounce mná with an "r" sound, as mná with an "n" sound is a perfectly acceptable pronunciation.