Combine sugar, eggs, lime zest, and juice in a medium nonreactive saucepan, and set over medium-low heat. Cook, whisking constantly, until mixture begins to thicken and holds the mark of the whisk, 10 to 12 minutes.
Remove pan from heat, and whisk in butter, a piece at a time, until well combined. Strain mixture through a sieve into a glass bowl. Lay plastic directly on surface to prevent a skin from forming. Chill 3 hours or overnight to set the curd.