Watch Will Ferrell Sing 'Danny Boy' on 'Letterman'

Jeffrey R. Staab/CBS /Landov
Will Ferrell sings "Danny Boy" while dressed as a leprechaun in honor of St. Patrick's day, on the Late Show with David Letterman, Tuesday March 17, 2015.

He's played an elf, a racing car driver and a deranged fashion executive. For Will Ferrell, performing "Danny Boy" dressed as a leprechaun is no great stretch.

That's exactly what the funnyman did when he got into the St. Patrick's Day spirit for an appearance on The Late Show with David Letterman

Dropkick Murphys Play Surprise Set at Boston's Logan Airport: Watch

"St Patrick's Day is a really serious holiday to me," he told Letterman. "We know of it as getting drunk and drinking green beer. But we forget in the 15th century, the real St. Patrick chased all the snakes out of Ireland. So, every St. Patrick's Day, I put on my outfit and walk the streets of New York educating people on the dangers of snakes." 

Will then put his energies into a whacky performance of "Danny Boy." "Oh Danny Boy," he sings, "watch out for snakes."