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
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
Tradition says that two trees grew up over their graves, entwining
together to form a Lovers Knot, standing guard over the site.