I think the third verse is actually :

We went out there by Castleknock
Says she to me, "Hang your coat on the lock"
Then I knew she was one of the rare old stock
From outside the zoological gardens

The 'rare old stock' being a lady of pleasure, who used to ply her trade there, and a coat (or other garment) was hung on the lock, covering the keyhole, for privacy in case the Garda took an interest