The beard, as the story clearly shows, is a sign of maturity and adulthood, and of the courage of warriors. In the Raids of the cows of Cooley is told how Irish warriors refuse to fight with C͹chulainn because he is beardless. […] To rob somebody of his beard was seen as a humiliation and a deprivation of the power of the warrior. Therefor the retention of the beard is articulated in several places in the legends by defeating the giant who claims the beards of knights to make a shirt thereof.
Koenraad Logghe in De Graal p. 147/8