The singer goes to Coolea "on a dancing expedition." After the dance and drinks his famous hat is missing. It had been worn by Brian Boru, Alfred the Great,... He searches all Ireland but, finally, a witch tells him it is in the Lake.
The tall tale nature of this song is shown by the two kings mentioned. Alfred reigned in Wessex (southwestern England) from probably 871 to c. 899. Brian Boru was born a copule of generations later, and died at the Battle of Clontarf in 1014. Thus it is chronologically possible that the hat could have passed on -- but there was effectively no contact between Wessex and Ireland at this time. Unless the Vikings captured the hat from Wessex and carried it to Ireland. But what are the odds of it surviving that? (Even if you assume it survived everything else). - RBW