In Irish mythology, Donn was known as the Dark One, Lord of the Dead. He was also known as the father of Diarmuid Da Duibne, who he gave Aengus Og to be nurtured. He was though of as the father of the Irish race.
He was known as the chief of the Sons of Mil ( the Milesians), a mythological race who invaded Ireland, driving out the Tuatha De' Danann.
According to myth, when he stepped on Ireland he was greeted by the lovely goddess Eriu. She asked him if he would honor her by naming the island that he was planning on conquering, in her name. Donn dismissed her and her request. He also scorned her. This was a bad mistake. Eriu prophesied that he would not step foot on Ireland again. The god of the sea, Manannan Mac Lir, caused a storm to engulf Donn and the Milesians and drowned Don of the coast of Ireland. This area became his final resting place and it is where Tech Duinn ( House of Donn ) is said to exist. Another version says that Eriu forbaded him to step onto the shores of Ireland therefore he lived on the rocky islet at the end of the Beare Peninsula.
This became the place where the dead went before they crossed over to the Underworld. Therefore he became the Lord of the Dead.
In Knockfierna, County Limerick was also Donn Firinne's residence. Cnoc Firinne ( the Hill of Truth ) takes it's name from Donn, who is said to forewarn the local people of bad weather by gathering up rain clouds around him on the hill. Here he is also known as the fairy king Donn of Knockfierna. There is a large earthern fort on his hill and a number of dolmens known as the 'Giants Graves' stand there today.
Sometimes he is seen riding on a white horse on stormy nights, when people would exclaim, "Donn is galloping in the clouds tonight!"