I just refer to Greek/Roman mythology because it's perhaps the most well known mythology (in any case, for me), but a parallel can be made with other mythologies. As a friend pointed out, Bran is the Celtic deity probably most connected with Mars. He was a warrior king who might be compared to the Greek hero Jason of the Argonauts, since he made many heroic voyages. It is interesting to note that he is also strongly connected with the Alder tree. In the Celtic calendar, each month (or sign of the zodiac?) is ruled by a different tree. Alder seems to have many Mars/Aries connotations. The most obvious is in the ancient Welsh myth "Cad Goddeu" (Battle of the Trees) in which the Alder fights in the front lines, being among the most courageous. Also in "Cad Goddeu" the warrior Bran had a branch of Alder, and the buds were set in a spiral that signifies rebirth. How appropriate for Aries! The Alder is considered to be fiery in its temperament, and when it is cut down it bleeds red - more Mars/Aries stuff.

I took a look at the Dictionary of Symbols (Jean Chevalier and Alain Gheerbrant) and there is stated that the function of the Roman Mars, in Celtic culture, is represented by two figures. One is Nodons (Irish: Nuada), the king-priest, who came from the warrior class but has a sacerdotal role, and the other one is Ogmios (Irish: Ogme), a God and Champion, Master of the single combat, of magic and of the Power of the Shadows. Of course, with the Roman Empire, those attributes had not much reason to be and they declined or were changed by the interpretatio romana.