Why not An mbheifeá ansin? In other words, why does the h drop out of mbheifeá?
Perhaps this is just a rule I need to memorize but I wanted to ask it outright because, as usual, it's just offered up here in the textbook without a word of explanation. Thanks!
The unmutated original form is "beifeá". You can still find it in: ná beifeá ansan = that thou wouldst not be there. (That's Munster dialect for Standard: nach mbeifeá ansin.)
In most circumstances "beifeá" is always mutated (eclipsed or lenited) due to particles before it: (Do) bheifeá ansin. = Thou wouldst be there. An mbeifeá ansin? = Wouldst thou be there?