Play video content EXCLUSIVE TMZ.com

If you've never heard the one about Casey Anthony walking into a bar by herself, buckle up ... 'cause it's a real knee-slapper.

Casey was drinking for one on Sunday at O'Shea's Irish Pub in West Palm Beach, FL right around happy hour time ... and oh, what a happy time she was having with the regulars.

In video obtained by TMZ, you see Casey trying to have a convo with a few dudes there -- we say trying, 'cause it mostly sounded like drunken gibberish ... with an Irish twist. Either way, it's clear Casey enjoyed herself for the hour or so she was there, even solo.

Unclear exactly what she was drinking, but it looks like a light brew to us -- maybe a Stella.