Green’s Dictionary of Slang

(Irish) a public lavatory.

1922		Joyce Ulysses 146: All kinds of places are good for ads. That quack doctor for the clap used to be stuck up in all the greenhouses. [Ibid.] 705: Theyre always trying to show it to you every time nearly I passed outside the mens greenhouse near the Harcourt street station.
1972		B. Rodgers Queens’ Vernacular 194: public toilet [...] greenhouse.