Big, great. Written More, Mor — the same Irish word, mór, in the spellings English gave it.
Big, great: it is the Irish mór. It is written More, Mor in English spellings of Irish names.
mór. English spellings of the same word include More, Mor, because the names were written down by ear.
Tullamore (Tulach Mhór), Ballinamore (Béal an Átha Móir).