She was a banshee who brought bad luck wherever she went. She could cause catastrophes and shapeshift into anything or anyone she chooses. But as powerful as she was, needed gold before Christmas Day or she would turn to tears and wash away forever. As she put it "Banshees are created from tears. Tears we were. Tears we are. And tears we shall be at the end." When she discovered Blarney Kilakillarney's gold she attemped to get him to give it away; she is not allowed to steal the gold for it doesn't count. When that didn't work she disguised herself as a beautiful fairy and trick his wife, Faye, into having Blarney give up his gold; saying that it is the source of greed. She complies and soon afterward Mag did the same to the other leprechauns, and they became shoemakers. Only Blarney refuses to give away gold, so she causes an earthquake which causes their piece of land to break off from Ireland; which becomes the island of Tralee. Soon afterward Lord Patrick, the lord of the leprechauns, tricks her into turning into tears and traps her in a pine cone from a tree "planted by St. Patrick himself" and in turn becomes a tree itself. He warned Blarney that should the tree fall or taken out of root, Mag would be released.
A little over a century passes and a young man, Dinty Doyle, dug out the tree and unintentionally sets her free. Later, she tracks down Blarney and slips "The potion of Generosity" into his tea so he give the gold away. But Blarney quickly gives his gold to Dinty instead. But while disguised as a girl named Collean she tricks Dinty into giving the gold her. Before Dinty can act when he realized what he had done, she puts him in a deep sleep of 100 Christmas'. However her efforts are in vain. Before she could even touch the gold the sun rises, and it's Christmas morning. Dinty and Leprechauns watch as Mag turns to tears and is washed away by the sea.