“We look like two Irish aunts,” Leary joked after Fallon brought out a side-by-side photo of the two stars, who are third cousins. “We look like sisters, that’s what we look like. Two old sisters.”

Leary and O’Brien both got their start in comedy, as a stand-up comedian and 'Saturday Night Live' writer, respectively.

Denis Leary reveals how he figured out his connection to revered TV host Conan O'Brien, who turned out to be distant cousins.