Okay I am sure I should understand this by now and if I dug around enough in the eclipsis/leinition explanations I could figure it out...but why is "bean" changed to "mbean" rather than to "bhean"? I honestly have read over the rules so many times but I can never remember them so I am more trying to develop an instinct for it, and my instinct would have said "bhean", maybe because I see that more often.
Feminine nouns like "bean" are lenited after the singular definite article - "an bhean".
But many prepositions, such "ar", "ag", "le" and others, cause eclipse of nouns after "an" - "ar an mbord", "ag an mbean", "leis an gcailín". This happens whatever the gender of the noun, and takes precedence over the lenition of feminine nouns after "an".
(Except in Donegal Irish, where they lenite instead of eclipsing).