Gallowglass mercenaries hired by Gaelic lords, and the planter families of the 1600s.
Two waves, three and a half centuries apart, and they could hardly be more different. The first were the gallowglass (gallóglaigh, foreign warriors) heavy infantry hired from the Norse-Gaelic west of Scotland. The first record of them is 1259, when Aedh Ó Conchobair, King of Connacht, received 160 Scottish warriors as a dowry. The MacSweeneys were settled by the O'Donnells in north Donegal; the MacDonnells, the MacCabes and the MacSheehys followed, planted by other Irish lords in other places. They were paid in land, and within a few generations they were Irish septs with Irish homelands like anyone else.
The second wave came to take land rather than be given it. From 1606 lowland Scots were settling north Down under Hugh Montgomery and James Hamilton; in 1607 Sir Randal MacDonnell put 300 Presbyterian Scots families on his estates in Antrim; and from 1609 the Plantation of Ulster proper brought British colonists onto land confiscated from the Gaelic chiefs who had fled in 1607. Stewart, Johnston, Graham, Bell, Craig, Thompson and Wilson are the names of that settlement. Graham and Johnston are Border names — reiving families the Crown was glad to see the other side of the water.
The two waves can look alike on paper, because Scottish Gaelic and Irish are the same language a few centuries apart. Mac Suibhne and Mac Dónaill are gallowglass names that read as native Irish; Ferguson and Morrison have Gaelic forms too. The lowland and Border names do not. Where a surname on this site has no Irish-language form, the page says so with a dash rather than inventing one.
Sources: Wikipedia — Gallowglass ↗ · Wikipedia — Plantation of Ulster ↗
Homelands are the historic heartland of the name, not where the family is now — people moved, and the Famine and the plantations moved them further.
A mercenary heavy infantryman from the Norse-Gaelic west of Scotland (gallóglach, 'foreign warrior') hired by Irish lords from the mid-thirteenth century. The first record is 1259, a dowry of 160 warriors to the King of Connacht. Paid in land, the gallowglass families became Irish septs: Sweeney, McDonnell, McCabe, Sheehy.
In two waves. Gallowglass families from the 1250s onward, settled by Gaelic lords as soldiers. Then Presbyterian lowland Scots from 1606 in Down and Antrim, and from 1609 across the planted counties of Ulster.
It is Scottish, and came to Ireland both ways — as gallowglass and as planters. In Ulster it was sometimes adopted as an English form of the native Mac Cathmhaoil, which is why some Campbells are Gaelic Irish by descent.
20: Adams, Beattie, Bell, Campbell, Craig, Ferguson, Graham, Johnston and more.
Most often Antrim (15), Down (10), Fermanagh (4), Tyrone (3).