Will Ferrell calls out Bill O'Reilly while dressed as a leprechaun

Will Ferrell celebrated St. Patrick's Day in style when he showed up on the Late Show with David Letterman dressed as a giant leprechaun.

While donning his all-green garbs, the comedian decided to call out a FOX News host with an Irish last name. Ferrell recalled seeing Bill O'Reilly at the SNL 40 special this past month, and said the news man didn't look like he was having a good time. Ferrell told Letterman that he was looking for audience reactions as the opening monologue began, and saw O'Reilly sitting there with his head rested on his fist, not smiling.

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"A real sourpuss," Ferrell said of the TV personality. "I don't know what he was doing there if he wasn't a fan."

Look back at the 40th anniversary show:

The Get Hard star, 47, also shared with Letterman why he decided to dress like a leprechaun, quipping that he takes St. Patrick's Day very seriously. "Every St. Patrick's Day I put on my outfit and I walk the streets of New York educating people on the dangers of snakes," he said.

That's when Ferrell went into his hilarious rendition of "Danny Boy."

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"Oh Danny boy, the snakes the snakes are calling," he sang. "From the Upper East Side and down from China Town. Don't walk the highline and stay away from Dumbo. Because Dumbo the snakes wear skinny jeans."