Michael Kelly (1762–1826) was a Cork-born composer, actor, and operatic tenor and onetime friend of Mozart’s, who ended his career running a combination sheet-music store and wine shop in London. Somewhere along the line, he invented what he called “Mr. Michael Kelly's sauce for boiled tripe, calf's-head, or cow-heel.” The sauce does indeed liven up simply cooked tripe, if you like that sort of thing, but is also an appropriate condiment for baked or boiled ham.
Adapted from “The Country Cooking of Ireland” by Colman Andrews
Mix the sugar, pepper, and mustard together in a medium bowl, then stir in the vinegar. When well blended, stir in melted butter. Serve warm.