According to my stepfather who is 63 and a half, therefore a mine of information, it comes from an american sketch from something similar to The Little Rascals where one of the characters was running away from a certain figure of authority and hid in a chicken coop, when the pursuer got to the chicken house he shouted:
"Is there anybody in here?"
to which the fugitive replied:
"There's no-one here but us chickens"
Over the years I have heard this anecdote adapted in to an Irish joke