The name Man is obviously of great antiquity as Caesar in his Gallic Wars refers both to Angelsey and Man as 'Mona' the latinised form that would correspond to the name Man. The description of the Island as in the middle of the sea clearly indicates Man. Pliny later distinguishes Man as the further Mona.
Many authors have attempted derivations of the name - Bishop Wilson in the early 18th century states that
THE Isle of Man, very probably, had the name it goes by now from the Saxon word mang (among), as lying almost at an equal distance between the kingdoms of England, Scotland, Ireland, and Wales; hence it is, that the neighbouring nations use the expressions Mancks-men, Mancks-language, etc.
However it obviously predates any Saxon invasion of these Islands - others have suggested that Man is cognate with the term 'Isolated' or 'Apart' suggesting that the use of the term in placenames such as the 'old Man of Conniston' are related. The use of 'man' to indicate cairns or similar objects in Norse settled parts of England would however to be unrelated deriving possibly from terms like 'false man' as a description of a cairn viewed from afar.
Irish place-name scholars now veer to the association of the names Man and Mannanan as both liked with water - Man would thus be understood as 'at water's edge'. This would explain both Man, Caesar's name for Anglesey and the use of Man or related placenames around the coast of the Irish sea. Kneen was of opinion that the name was cognate with hilly or mountainous.
W.W. Gill in an essay on the name 'Man' in Gaelic Literature and Topology brings in many of uses of the name.