I have a vague recollection of seeing signs such as this one many years ago. This one is in a lane often known to have some art along its length so I would guess that this sign is 'art'. Especially so as it looks to be a friendly sign.
When I took the photo, I did not realise how wonderful the brick work is.
Can I remember where the lane is? Nope. Probably off Little Bourke Street.