"The violent rejection by the other women of Derbforgaill, the woman who can melt snow like a man, ultimately rests as much on the heat as the amount of the urine. With her Galenic humours, theories of heat as the prerogative of male bodies and moisture of women's, are confounded; Derbforgaill is dangerous, both as the woman from outside the group and also as the woman with the subversive body who might be capable of both giving and experiencing pleasures in sex in ways that usurp a long-standing male prerogative and disturb the standard of gender by which women themselves collectively orient their gender identity."
-- Ann Dooley, Playing The Hero, p. 180
So, this definitely isn’t what Dooley is going for, but what I’m getting out of this analysis is that a trans reading of Derbforgaill would be fascinating. Her whole story is about the fact that she’s called upon to participate in a urination game (they didn’t have Netflix in those days), which she hesitates to do “because she was not a lewd woman”. When she does as she’s asked, she immediately faces violence for it from the women who put her in that position in the first place, because her body is unlike theirs. If we read Derbforgaill as a trans woman, it adds a whole new layer to the other women’s rejection of her based on her ability to urinate in a particular way which they themselves are unable to achieve earlier in the scene.
(It’s also then interesting to me that Cú Chulainn, as an ambiguously-gendered figure, is the one to come to her defence. This is meaningful even without a trans reading of either of them -- both are positioned as outsiders -- but that gender element adds an additional layer to the interaction.)