Maris is an element appearing in multiple Celtic languages, meaning "great", and is used as a suffix in the name Onomaris, according to Celtic culture: a historical encyclopedia. Vol. 1-, Volume 2 edited by John T. Koch, p.1396.
Maris hasn't any connection with the god Mars. In Latin, Maris is the genitive for of mare (sea), and the genitive form of Mars is Martis (hence the adjective "martial"): there is a -t- in the radical of the god's name.