Oisín is a character from Irish mythology, known as a poet and warrior. He is the son of Fionn mac Cumhaill, the leader of the Fianna, a group of legendary warriors. Oisín is famous for his adventures, particularly his journey to the magical land of Tír na nÓg, where he fell in love with Niamh, a beautiful fairy princess. In Tír na nÓg, Oisín lived for many years, enjoying eternal youth and happiness. However, he eventually returned to Ireland to find that centuries had passed. His story highlights themes of love, loss, and the passage of time in Irish folklore.