The deaths of Efnisien and Branwen in the (much later) second branch
of the Mabinogi suggest themselves as rather remote parallels to the
death of Derdriu in the Exile of the Sons of Uisliu. Efnisien's death
is evidently intended to be a suicide; that of Branwen, who died of a
broken heart, is effectively so; and their deaths, like that of
Derdriu, can be seen as acts of heroism as well as tragic necessities
within the mercilessly rigid and destructive heroic world they
inhabit. The second branch ("Branwen") is, by the way, both the
most heroic and archaic, and the most Irish, branch of the Mabinogi.
P.S. This is a strictly amateur opinion and may bear no relation to
the "professional" sense (if there is one) of the incident's meaning.