Recently, I've been listening (for the first time) to a box set of Pete Seeger. Since a lot of the songs are folk songs, I've heard other versions of some of them. So while "Molly Malone" wasn't a new song to me, because I recently listened to Seeger's version, I got to thinking about it and realized that it shares some similarities with the Decemberists' "Eli, the Barrow Boy" from Picaresque.
The first lines of both mention a city. "Molly Malone" names Dublin specifically ("In Dublin's fair city") where "Eli, the Barrow Boy" has just a general "town." I'd always heard it as "Eli, the barrow boy of the old town," but according to the Decemberists' website, it's "Eli, the barrow boy, you’re the old town." It's a minor point, but both start with a location.
Both characters "cry" out the wares that they have to sell. The second verse of "Molly Malone" explains that "She was a fishmonger," and the "Cryin' 'Cockles and mussels, alive, alive, oh'" line is at the end of every verse and acts as a transition to the chorus (which is comprised of all the same words). While Eli has a wider variety of merchandise (he "Sells coal and marigolds" and "Corn cobs and candlewax"), he too "cries out all down the day."
It's not as obvious in "Eli, the Barrow Boy" as it is in "Molly Malone," but both have a fair amount of repetition to describe how much time the characters spend trying to make sales. "All the day," "All down the day," or "Down the day" appear (usually twice) in every section of "Eli, the Barrow Boy," and "Through streets long and narrow" (or, as some versions have it, "Through streets broad and narrow") is the second-to-last line in every verse in "Molly Malone."
While Molly Malone dies of a fever and Eli drowns, both return as ghosts to continue pushing their wheelbarrows through the streets and sell their goods. The last verse of "Molly Malone" explains that "Now her ghost wheels the barrow / Through streets long and narrow," and - likewise - the last section of "Eli, the Barrow Boy" has Eli saying, "But I am dead and gone and lying in a church ground / But still I push my barrow all the day."
I know Colin Meloy has some interest in folk songs (evidenced by the Colin Meloy Sings Shirley Collins EP and The Hazards of Love album), so I'm assuming that "Molly Malone" did have at least some influence on "Eli, the Barrow Boy." I think the similarities would be too much of a coincidence otherwise.