Apparently, way back when some of the oldest dioramas were being put together, the artist who was painting the backgrounds painted in these leprechauns in lieu of a signature. There are various stories about how the museum felt about this at the time, but nowadays they seem to treat it as a cute joke that they don't try too hard to keep secret.
The information desk hands out a sheet of clues to tell you where to look for them, so off we went. Some of them were very hard to find, and several were inaccessible due to some halls being renovated. We were successful in spotting several of them though. Although we all spent time hunting for leprechauns, I think Trey got the biggest kick out of it. We were also able to find the hidden Yoda in the Imax Theater mural.