Andrea Casiraghi is dressed in formal clothes for the religious ceremony that was part of the royal wedding of Prince Albert II of Monaco to Charlene Wittstock on July 2, 2011. This was in the main courtyard at the Prince's Palace in Monte Carlo, Monaco. Andrea Casiraghi was born on June 8, 1984, so he had just turned 27 when this photo was taken.
We are going to take a wild and uncertain guess and say that that's his mother, Princess Caroline of Monaco, in the giant hat to his right (our left). The arguments in favor of it being Caroline are that he would naturally be walking with his mother at such an event, and the hat and dress she's wearing appear to be that of a more mature woman, not a date. The argument against is: would Princess Caroline, sister of the monarch, wear such a frilly outfit to a solemn religious occasion? We don't know. The Monagasque guards surrounding them wear those unusual floppy helmet frills, so perhaps in Monaco, anything is possible.