American Gods: What was the deal with Mad Sweeney's wrong coin?

In American Gods, Mad Sweeney is some sort of leprechaun (the large old intimidating sort that would kick yer fookin arse). When he first meets the book’s protagonist, Shadow, he shows off a bit by pulling a gold coin out of thin air and gives it to Shadow.

Shadow tosses this coin onto the casket of his dead wife as she’s being buried, and she later comes to life, though progressively rotting a bit, with the implication that it was the coin’s doing.

And later on, Mad Sweeney shows up in a panic, asking for the coin back, babbling that it was the wrong sort of coin he had given Shadow: it should have been reserved for royalty, to be given to the King of America if there were such a thing. When Shadow tells him that he didn’t have the coin any longer, Mad Sweeney is driven to despair and winds up killing himself.

So was this just an elaborate plot device, or is there some actual legend or other factor connected to the fabled coin?

I don’t quite remember it that way. I think the gold coin was something that Mad Sweeney used to keep himself alive, despite no one believing in “little people” anymore. IIRC, Sweeney only meant to give Shadow a simple, non-magical gold coin to toss into the grave (reflecting the ancient Hellenistic Greek tradition of sticking a coin in a corpse’s mouth - as a payment for Charon the Ferryman. But Sweeney (and Shadow) was stinking drunk when he gave it to him. Shadow almost jokingly tossed it into the grave, but it resurrected his wife as a revenant. It was only after Shadow had done that that Sweeney realized what he’d done. He made an attempt to retrieve it, but since it was already in the grave it was too late. Hence, Sweeney simply died without his magical coin.

I found the text online, and it seems to confirm what I remember: that Mad Sweeney didn’t regard the coin as his property to give away, that he would be in a huge amount of trouble for doing so, and he was so despondent over the fact that he actively chose to kill himself.

It was at this point Mad Sweeney begged Shadow for twenty dollars so that he could get “a ticket out of here,” which turned out to be a bottle of alcohol that would aid him in drinking and freezing to death.

The coin was the Sun.

Or at least, the inner Sun, the fire of life, and the Hermetic Sun of alchemy that turns base into gold - the Philosopher’s Stone, which co-incidentally also gives life.

It’s the little lightpump at the heart of the rainbow, that creates pots o’gold.

Compare with when Zorya gives Shadow the Moon as protection. Lovely dualism yet reversal of the traditional Man=Sun:Woman=Moon roles there - Shadow has the Moon, Laura has the Sun.