Y Gwyll is a Welsh TV program whose name means The Dark... Where does the dy in the adjective come from, or is it just a coincidence?
tywyll - dark as in nos dywyll ('a dark night'). tywyllwch - darkness.
gwyll - dusk, gloom, twilight. This is probably what is inferred in the drama's title - the content is constantly referring to past mis-deeds and hints at shadowy goings-on, etc
The GPC (a Welsh etymological and historical dictionary) mentions the possibility of a shared element in the origins of tywyll and gwyll.
No, not in Modern Welsh, anyway. But it is even possible that gwyll came about thorough a loss of the ty- from tywyll somewhere along the line. We are probably some 1500-ish years too late to be sure!