Make the simple syrup by combining the cranberries, sugar, water and orange peel in a small saucepan.
Set over medium-high heat and bring to a boil.
Simmer for 5 minutes until the cranberries have burst then allow to cool completely.
Strain out the cranberries and the orange peel.
The syrup can be kept in an airtight jar in the fridge for up to a month.
To make the margaritas, combine the cranberry syrup (I start with ½ cup and add more to taste), tequila, Cointreau, lime juice and ice in a cocktail shaker and shake well.
Alternatively, blend all the ingredients together in a blender for a frozen margarita. You can make this in a large pitcher too if you're serving a crowd.
To rim the glasses, use a lime wedge and wip the edge of the glass then dip into salt.
Pour the margarita mixture into the glasses and add limes and cranberries then serve.
Garnishing with rosemary sprigs will add even more Christmas cheer but is optional.