The Book of the Dun Cow (12th century)

Mermaid in a C14th Welsh manuscript (not the BotDC I'm afraid). CC image NLW.

A monster of the deep was once caught in the nets of the fishermen of Bangor, if we but credit the Leabhar-na-h-Uidhre [the12th century Book of the Dun Cow], which tells the following wild legend:-

"Eochaidh, from whom Lough Neagh derives its name, was drowned in its eruption, together with all his children except Liban, Conaing, and Curnan. Liban was preserved from the waters of Lough Neagh for a full year in her grinan (palace) under the lake. After this, at her own desire, she was changed into a salmon, and continued to traverse the sea till the time of St. Comgall, of Bangor.

It happened that Comgall despatched Beoan, son of Innli, of Teach-Dabeog, to Rome, on a message to Gregory, to receive orders and rule. When the crew of Beoan's curach were at sea they heard the celebration of angels beneath the boat. Liban (mermaid) thereupon addressed them, and stated that she had been three hundred years under the sea, adding that she would proceed westward and meet Beoan that day twelve-months, at Inbher Ollarba (Larne, but perhaps it should have been Inver Ollair, the mouth of the Six-Mile Water at Lough Neagh), whither the saints of Dalaraidhe, with Comgall, were to resort.

Beoan, on his return, related what had occurred, and at the stated time the nets were set, and she was caught in the nets of Fergus, of Milliuc (Meleeg, in the civil parish of Camlin, Co. Antrim), upon which she was brought to land, and crowds came to witness the sight, amongst whom was the chief of Ui Conaing. The right to her being disputed by Comgall, in whose territory - and Fergus, in whose net - and Beoan, in promise to whom she was taken, they prayed for heavenly decision; and next day two wild oxen came down from Carn-Airend (Carnearny); and on their being yoked to the chariot on which she was placed, they bore her to Teach-Dabeoc, where she was baptised by Comgall, with the name Muirgen - i.e. 'born of the sea' - or Muirgelt - i.e. 'traverser of the sea'."

This (from the Dun Cow, but with added suggestions of placenames) is in 'An Historical Account of the Diocese of Down and Connor' by the Rev. James O'Laverty, v2, 1880.