Shamrock and Roll

Shamrock and Roll is a 1969 Merrie Melodies cartoon from Warner Bros.-Seven Arts.

Plot
When Merlin the Magic Mouse and Second Banana travel to Ireland via magic carpet and crash-land in a shamrock patch, they come across a crazy old leprechaun named O' Reilly. When the mystical creature doesn't believe Merlin can do magic tricks, the magician rodent performs for him. Then thanks to his OWN magic, O' Reilly steals Merlin's pocket watch, resulting in a crazy chase to get the watch back, ending with a collision course on Big Ben!

Trivia
This is the only Merlin cartoon where he got beaten by his opponent.