St. Aaron was born in the Sixth Century somehwere in the British Isles. He was the abbot of a monastery located on the island of Cesambre, which is near the former town to Aleth, in Brittany, France. This island was hard to get to, apparently -- it was accessible only at low tide.
His claim to fame was the warm welcome he have to Saint Malo on his mission to France, and worked closely with him thereafter He died some time in the sixth Century of natural causes. For a time, the island bore his name, Aaron, until the town of Aleth was deserted when the bishop of Aleth (John de la Grille) moved his see to the island in 1150. Now the island is the town of St Malo.
St. Aaron's feast day is celebrated by the Catholic Church on June 21, though usually only locally in Brittany.
There's another St Aaron, remembered on October 9, who was a monk a Cluny and a student of Saint Odilo. He was the first abbot of the Benedictine abbey at Tyniec, Poland, and was ordained first archbishop of Krakow in 1046. He died in 1059 of natural causes.
And apparently there's also a Saint Aaron of Caerleon. the only information I could find on him was that he worked closely with Saint Julius, and that he was martyred in the year 303 in Wales under the persecutions of the Roman Emperor Diocletian. Nonetheless, his feast day is celebrated on July 3.