Sadhbh in Séadna addresses someone as "a bhitheamhnaigh na croiche duibhe!", and Norma Borthwick translates it as "thief of the black gallows!" - as if such a phrase existed in English. [I think Norma Borthwick edited Séadna; she was at least the inspiration behind its publication, and so her translation is quite authoritative.]

What is a thief of the black gallows? A thief who ought to go to the gallows? A downright scoundrel? Are gallows even black anyway?

Perhaps related to the folk tale:

http://en.wikipedia.org/wiki/The_Tale_of_the_Shifty_Lad,_the_Widow%27s_Son

I think the character is saying to the bithiúnach that he is the sort of person who would end up on the gallows, a jailbird, a thoroughly disreputable character. The gallows may have been black but dubh is probably used for anything particularly bad or sinister.