The MacNeills, like the MacEwens, were descended from Aodh Anrothan O'Neill (Hugh the Solitary), Prince of Ulster, who left this position in 1038 AD, moved to Scotland, and married the heiress of Cowal and Knapdale. Anrothan himself was descended from a very long line of High Kings of Ireland, including Niall of the Nine Hostages (357-365 AD).

The MacNeills of Knapdale held Taynish on the west side of Loch Sween, as well as the Island of Gigha off the coast of Kintyre. As hereditary Keepers of Castle Sween, they were of great importance in the area.

The following is a poem written by Effric Neyn Coreitill (MacCorquadale), in 1431, lamenting the death of her husband, Hector Torquil MacNeill of Castle Sween. It was originally written in Gaelic:

Rosary, though kindliest sorrow,
- though are ever my delight,
telling of the noble bosom,
where I lay until tonight.

Death has filled me with its sadness,
where is the arm I clung to long?
Ah! I saw it not departing:
His the valiant and the strong.

Joyful voice of softest music,
known, it everywhere remains,
Lion of Mull of the White Towers,
Hawk of Islay of Smooth plains

There is no joy among our women,
At the sport, men are not seen;
Like the skies when winds are silent,
so with music is Dun Sween.

On Clan Neill, they've taken vengeance;
See the Palace of the Brave!
Cause to us of sad lamenting,
til they lay us in the grave.

In the early 1400's, Alexander MacMillan married Erca, the daughter of Hector Torquil MacNeill, 1st of Taynish and Constable of Castle Sween. As a dowry, Erca brought to the marriage ten farms which covered the area extending from Castle Sween to CeannLochCaolisport. After their marriage, Alexander and Erca went to live at Castle Sween, and a new section was built for them, which is still known as "MacMillan's Tower". The Constableship of the Castle thus passed to the MacMillans.

In 1700, the MacNeills of Crear (an estate on the east side of Loch Caolisport, near Ormsary estate), exchanged their land for the Islands of Colonsay and Oronsay, with Crear going to the Campbells.

The area comprising Erca's dowry (which includes Ballimore, South Knapdale) was purchased by Major General Sir Archibald Campbell, K.B., in 1776.

To the west, at Keills, the proprietor in the early 1800's was a MacNeill of the Island of Gigha. By the mid- 1800's, in the face of Campbell expansion, the MacNeill power had faded, to 2 farms in North Knapdale, and one (Drimdrishaig) in South Knapdale.

As the Gigha and Knapdale MacNeills lost their strength, that of their kin on Barra, in the Outer Hebrides increased, so that now, the chief of the clan is officially MacNeil of Barra.