According to Irish mythology, Donn, or the Dark One, is the Lord of the Dead and father of Diarmuid Ua Duibhne, whom he gave to Aengus Óg to be nurtured. Donn is regarded as the father of the Irish race; a position similar to that of Dis Pater and the Gauls, as noted by Julius Caesar (the Roman emperor).

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.

In modern Irish, the word for the colour brown is "donn".

References

Dictionary of Celtic Myth and Legend. Miranda Green. Thames and Hudson Ltd. London. 1997