Combine the chopped rhubarb, sugar and water in a pot. Bring to a boil and simmer for 20 minutes. Strain and pour into a sealable container. Store in the refrigerator until ready to use.
To make the margarita, combine tequila, Cointreau, simple syrup and lime juice in a cocktail shaker with ice. Shake well to combine. Pour over ice in a lowball glass to serve.