According to legend, when Donn and his band of Milesians (the aforementioned Sons of Mil) arrived on the shore of Ireland, they were greeted by the lovely goddess Eriu. She inquired of Donn whether she might have the honor of lending her name to the island that the Milesians planned to conquer. But Donn dismissed her and her request, scorning the goddess. This was a mistake, and Donn was made to pay for it. For as a result, the god of the sea, Manannan Mac Lir, caused a storm to engulf the Milesians, which, in some versions of the story, drowned Donn off the coast of Ireland. This area became his final resting place, and it is where Tech Duinn is said to exist.
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Dictionary of Celtic Mythology
This book is a great source for information about Celtic mythology! Described as both a "who's who" and a "what's what", this reference book features entries on the important gods, heroes, and other characters from Celtic myth and legend. And there is a good entry on the god Donn.