"Serpentine avenue" is a street in Sandymount, a little south and west of where Stephen is walking on the Sandymount strand in Proteus. On the map at right, Serpentine is the street at the far lower right which passes under the Dublin & Kingstown railway line and becomes the Tritonville Road after the intersection with Claremont Road.
Stephen mocks himself in Proteus for praying "to the devil in Serpentine avenue that the fubsy widow in front might lift her clothes still more from the wet street." A "fubsy" person is chubby and squat.
But the widow aroused Stephen in some way that he is still seriously thinking about. In Circe he weaves the memory into his prophetic dream of Haroun al Raschid: "It was here. Street of harlots. In Serpentine avenue Beelzebub showed me her, a fubsy widow. Where's the red carpet spread?" Stephen (or whatever hallucinatory version of his consciousness the episode is presenting us with) seems to be expecting to meet his widow in the red-light district, fulfilling the promise of the man in the dream (recalled in Proteus) to introduce him to someone important: "In. Come. Red carpet spread. You will see who."