Green’s Dictionary of Slang

1. the penis.

1790

‘Bumper Allnight. Esquire’ Honest Fellow 11: Then master O’Blaney / Pull’d out his Shillaley / / (A weapon he never shew’d women in vain).

2. (Aus.) in horse-racing, a whip.

1996

J. Byrell Lairs, Urgers & Coat-Tuggers 126: If his conveyance was inclined to loaf or had a tendency to ‘drop off’ during a race, the wrong time to do that was with ‘The Darb’ up. He could really wield the old shillelagh.