This is just a guess, but I believe "Adhémar" is a varient of Irish "Aodhamair" (which has the older, archaic spelling "Aedammair").
The Irish-Gaelic root is definately 'aodh' meaning 'fire.' I do not know the origin of the 'mair' part.
This name is typically feminine, though I once read a king of Connaught used a variation on it.
If I am correct in connecting "Adhémar" and "Aodhamair," its French attribution is not too bizarre (akin to Ronan, Fiacre, etc). But I too can only make guesses for pronunciation (the 'h' is throwing me off).
Very rough Gaelic pronunciations are: Aodhamair- [EE-mur] Aedammair/Aedhammair- [AYM-mur]