Plain. Written Magh, Moy, Maw, May — the same Irish word, magh / maigh, in the spellings English gave it.
Plain: it is the Irish magh / maigh. It is written Magh, Moy, Maw, May in English spellings of Irish names.
magh / maigh. English spellings of the same word include Magh, Moy, Maw, May, because the names were written down by ear.
Armagh (Ard Mhacha), Omagh (An Ómaigh).