Drain the lychees juice into a pan and put the lychees in a large serving bowl. Add the lemongrass and sugar to the pan, heat gently until the sugar dissolves, then boil for 1 minute. Turn off the heat and set aside – the lemongrass will add flavour as it cools.
Strain the syrup over the lychees and tip in the fruits. Chill. Serve with macaroons.