I have been researching the Downie surname and have published the results at http://downiesurname.yolasite.com/http://downiesurname.yolasite.com/. While completing the research, I have identified that there are a number of possible derivations of the surname Downie that arise in the western or highland area of Scotland, as opposed to the Downie families of eastern Scotland (Barony of Downie).
There is a probability that the Downie surname in the south west/west of Scotland has derived for the original surname Mac Gille/Maol Domhnaich to McAldonich, Mcildonich, and Mcoldonich which later developed into surnames such as Mcildownie, Mcgildownie, Mcildonie, Mcgildonie, Mcildounie, Mcgildounie and Mcillduine (and various minor corruptions of these, i.e. Mcaldownie, Mcildowney, Mckildownie or Mcildony) Dounie, Donie and finally to Downie.
From reading there appears to be a number of sources of the Downie surname in Scotland and Ireland, with the intermittent mix in Ulster, that with DNA testing we should be able to identify the different evolution of the Downie/Downey surname. There is a Downie/Downey DNA project at Family Tree DNA http://www.familytreedna.com/public/downey/default.aspx?sect..http://www.familytreedna.com/public/downey/default.aspx?sect...
The following sources are not exhaustive, but include:
•Geographical, there are Dounie/Doune/Downie place names or hill forts or Dun in most parishes in Scotland, including Aberdeen, Angus, Stirling, Perth, Inverness and Isle of Lewis; •Geographical, from the Barony of Downie in Angus; and •As a derivative of the Gaelic Mac Gille/Maol Domhnaich "son of the servant of the Lord (Sunday)" or McAldonich which both are anglacised to Macgildownie, Mcildownie and Gildownie (and many variations) to Downie, mainly in the parishes of Argyll, western Perth and Inverness.
•O’Dunadhaigh is a person identified with a fort or Dun. This surname is found mostly in County Galway, South west Cork and Leinster. The surname is Anglacised to Downey; •Mac Dunadhaigh, identified with a fort or Dun this is the surname of an old Galway family; and •O’Maoldomhnaigh from a church or Sunday, this name is anglacised to Moloney (sometimes Muldowney) and is the name of a Dalcassian family who were chiefs of the Barony of Tully, County Clare and the name is still common in Munster.
In Northern Ireland (Ulster):
•Mac Giolla Domhnaigh, "son of the servant of the Lord (Sunday)" which is an Ulster name and is anglacised to MacGildowney, Gildowney, Mcildowney, McEldowney, McDowney and Downey. The Scottish form of the same surname is spelt Mac Gille Domhnaich.
•The first name such as Maol Domhnaich, or Muldonich meaning the "Lord's Devotee" have been used in Scotland and may have been the precursors to the Mac variations.