The mention of "the steps from Leahy's terrace" fixes Stephen's location at the beginning of Proteus fairly precisely, and also links him with the location of Bloom in Nausicaa. Leahy's Terrace is a road in the southeastern suburb of Sandymount. It runs SW-NE, beginning at Sandymount Road and terminating, at the time of the novel, at a seawall bordering Sandymount Strand. (Since that time, land has been reclaimed from the sea, pushing the seacoast further east.)
Stephen is walking on the sands exposed by the ebb tide and sees two women coming down the steps from the road to the beach. The Star of the Sea Church that focuses the action in Nausicaa is also sited on Leahy's Terrace, very near the water. It seems certain, then, that Bloom and Gerty MacDowell cannot be seated far from where Stephen is walking when he sees the two old women descend the steps for their walk on the beach.
The steps that Stephen looks at no longer exist, and their former location is a matter for detective work. In 1960, before the Irishtown reclamation project, William York Tindall published a photograph of steps that he assumed were the ones mentioned in the book, and Clive Hart and Ian Gunn reproduced his image in their James Joyce's Dublin (2004). But they have discovered more recently that a smaller section of the strand was reclaimed in the 1920s, meaning that Joyce's steps must have been located still farther west, closer to the church. See Gunn's short article in James Joyce Online Notes.