Originally, Donn was the chief of the Sons of Mil, a mythological race who invaded Ireland, ousting the Tuatha Dé Danann. Donn slighted Ériu, one of the eponymous goddesses of Ireland, and he was drowned off the south-west coast of the island. A place near this spot, on a small rocky island named 'Tech nDuinn'. ('the House of Donn'), became Donn's dwelling place as god of the dead. This house was the assembly place for the dead before they began the journey to the otherworld.
Cnoc Fírinne, County Limerick was Donn Fírinne's residence. Cnoc Fírinne (meaning 'Hill of Truth') takes its name from Donn, who is said to forewarn the local people of bad weather by gathering up rain clouds around him on the hill.
Dictionary of Celtic Myth and Legend. Miranda Green. Thames and Hudson Ltd. London. 1997