It's actually a double genitive: the genetive statement Mí Mheán Fómhair also contains the genitive statement Meán Fómhair. When you make a genitive out of a genitive statement, lenite the first word of the original genitive statement. Thus you have: meán + fómhar = Meán Fómhair (gen. 1), then mí+(Meán Fómhair) = Mí Mheán Fómhair (gen 2)
Is it just me, or are Mheáin and Dheireadh pronounced as if Mh and dh aren't there? I'm hard of hearing from birth and lipread a lot, so seeing someone speak these would help A LOT; but without that, can anyone at least explain how these sounds are made? Le do toil agus go raibh maith agat!