The following is one of my favorites of theirs, though I doubt many would argue it's their best, whatever that means. I'm just charmed by the sweet, wistful romanticism of the song, and have been since before I started grade school.
I'm pretty sure the above was the version recorded on the album "The Boys Won't Leave the Girls Alone." Speaking of which, the title song is probably the most famous of their songs. I'm pretty certain the song is traditional, so it's not really fair to say others have "covered the Clancy Brothers' song," because it's not theirs. Nevertheless, I'd be willing to bet that the majority of the many, many musicians who have played this song first learned of it directly or indirectly from the Clancy Brothers and their recordings. I hear this played once or twice a week, but never one of their versions... perhaps this post may introduce this group, however sappy they may seem by today's standards, to some of my young friends.
Again quoting The Guardian,
"The best ballad singer I ever heard in my life" was Bob Dylan's verdict on Liam Clancy, who has died aged 74. He was the last remaining member of the best-known of all Irish folk groups, the Clancy Brothers and Tommy Makem, who made an impression that was strong enough for them to break through into the mainstream. The popularity of the quartet, comprising Liam, his older brothers Tom and Pat and family friend Tommy, was unrivalled in the 1960s, especially in the US, where the four men had settled. With their hard-living, hard-drinking image, singing Irish folk songs in a hearty and rousing style, the Aran-sweater-clad Clancys inspired Irish bands of all musical genres. Among those who have claimed an influence are Sinead O'Connor, the Pogues, Bono of U2 and the Dubliners.Yeah, they really were all that. Liam, I am saddened I no longer share the planet with you.