He was just 25 years old. On this day in 1916, Seán Heuston was executed by British forces at Kilmainham Gaol for his role in the Easter Rising. A railway clerk turned rebel leader, Heuston commanded a small outpost at the Mendicity Institution and held off hundreds of British troops for over two days — giving crucial time to the main rebel forces. After surrendering, he was court-martialed and sentenced to death. In his final hours, he wrote a calm, forgiving letter to his family and…