`While awaiting the completion of his weapons on the Isle of Man, Conchobar was able to overpower a mermaid. To induce him to liberate her, she informed him that she was Teeval, the Princess of the Ocean; and promised that if he caused the smith Culann to form her representation on the shield surrounded with this inscription, 'Teeval, Princess of the Ocean,' it would possess such extraordinary powers that when ever he was about engaging his enemy in battle, and looked upon her figure on the shield, read the legend, and invoked her name, his enemies would diminish in strength, while he and his people would acquire a proportionate increase in theirs. Conchobar had the shield made according to the advice of Teeval, and, on his return to Ireland, such extraordinary success attended his arms, that he won the kingdom of Ulster.
Source: https://www.sacred-texts.com/neu/celt/fim/fim04.htm#Celtic