slang. The phrase «to have a downon» (see Down) is often varied to «have a derry on.» The connection is probably the comic-song refrain, «Hey derrydown derry.»
1896. `The Argus,' March 19, p. 5, col. 9:
«Mr. Croker: Certainly. We will tender it as evidence.
(To the witness.) Have you any particular `derry' upon this Wendouree? – – No; not at all. There are worse vessels knocking about than the Wendouree.»