MacDermot was chieftain of Moylurg, a Celtic Kingdom in North Roscommon. He had a beautiful daughter, Úna Bhán - so named because of her long blonde hair. His neighbour was Tomás Láidir Costello, a good and sincere man, handsome and strong.
Úna Bhán and Tomás Láidir fell in love but MacDermot would not allow the relationship because he believed Tomás Láidir was not good enough for his daughter. Tomás Láidir was banished from the area and MacDermot had Úna Bhán confined on Castle Island, Lough Key, then called "The Rock".
Úna Bhán went into a deep melancholy and was dying of grief. Tomás Láidir, hearing of the situation went to see her, and when he left, vowed that if MacDermot did not send a message for him to return before he reached the river, he would never go back. The messenger was sent, but did not reach Tomás Láidir until after he had crossed the river. Being a man of honour Tomás Láidir was unable to break his vow and did not return.
Úna Bhán died of a broken heart and was buried on Trinity Island, Lough Key. In his grief Tomás Láidir used to swim to the island every night to keep vigil at her grave. Eventually he got pneumonia, and realising that he was dying requested that MacDermot allow him to be buried beside Úna Bhán. His request was granted and so the two lovers were belatedly united.
Tradition says that two trees grew up over their graves, entwining together to form a Lovers Knot, standing guard over the site.