Knowing the Gaelic form of our surnames is a very helpful in genetic genealogy research. There are many surnames in Ireland and Scotland that include the name that we say in English as "John." However, that is not entirely true as the etymology of many of the "John" surnames are not from English, but Gaelic, taken from the Latin name Iohannes, which in turn is a loan word name from the Hebrew Yohanan.
Surnames in the "John" category include Johnson, Johnston, Johnstone, Jones, Jackson, MacCain, MacCane, MacEoin, MacOwen, MacKane, MacKean, MacKeen, MacKeon, MacKeone, MacKeown, and MacShane. All these surnames come from the Latin Iohannes by one route or another. I discovered this complexity when researching my own surname early in my McCain genetic genealogy project.
You would think the etymology of the surname McCain would be simple enough, but it is not. If you open a standard Irish or Scottish surname book, it normally says McCain is the anglicized form of the Scottish name Mac Iain. This is usually followed by a reference to either the Glencoe or Ardnamurchan Mac Iain families of Scotland. What these books do not tell you is there were several dozen McCain families scattered throughout the Gaelic speaking world.
To complicate matters, there are several Gaelic surnames that have been anglicized as McCain. A list would include the surnames Mac Eáin, Mac Catháin, Mac Aodháin, Mac Caodháin, and Ó Catháin. Notice the surname book version, Mac Iain, is not listed. This is because Iain and Mac Iain are late entries into the Gaelic world. In the case of my own family, I knew the Gaelic form fortunately, which is Mac Eáin.
Eáin (said Ane) is a variation of the common Gaelic name Eóin (said Owen). The Eáin form is peculiar to Argyll and the southern Hebrides and it is considered Scottish Gaelic in origin. The surname was introduced into Donegal by Argyll Gaels that settled there in the 1500s. Eóin appears in early medieval Irish sources and is the Gaelic form of the Latin name Iohannes, or as English speakers know the name, John. Iohannes is itself a loan word to Latin from the Hebrew Yohanan (in full y'hohanan) meaning “Jehovah has favored.”
As you read through the Gaelic manuscripts you will not find any evidence of the variant Eáin form of Eóin until 1499 AD. That year in the Annals of Connacht one writer spelled the name of Eóin Mór Mac Dónaill, King of the Isles, as Eighín Mor Mac Domnaill, with Eighín being a slender vowel form of Eáin. This Eóin Mór Mac Dónaill was the chief of the McDonalds in Argyll, Islay and north Antrim. From that time on you can find the name sporadically written to reflect this Eáin form, but Eóin was still the dominant written form in literary Irish regardless if the name was said Eóin or Eáin. Literary Irish was the Gaelic used in both Ireland and Scotland by the educated classes.
By the mid-1600s, the Mac Eáin variation had its own spelling in literary Irish. The Scottish Mac Mhuirich family of Islay, historians and tradition keepers for the McDonalds, used several similar spellings of the surname when writing in the mid-1600s. In their Red Book of Clanranald, they spelled the surname mac Ceaain, Mac Eaain and Mac Ea'ain. The mac Ceaain spelling is an abbreviated form of the longer Mac Mhic Eaain, a clan surname form meaning “son of (the) son of Eáin.” This form functions much like the Ó does in Gaelic surnames, especially Irish ones. I asked the Sabhal Mòr Ostaig, which is the Gaelic college on the Isle of Skye, for the current spelling in Scot’s Gaelic and they gave me Mac Eain, with no accent over the “a.” Mac Eáin is the modern Ulster Gaelic form.
The Lallans spellings also demonstrate the pronunciation shift from Mac Eóin to Mac Eáin by the 1500s in Argyll. Lallans spellings of Mac Eáin from that time were McAne, McEan, Makayn, Mcayne, McEan, and McKean. In my research, I made an interesting discovery. In the 1400s, the northern Spanish and Portuguese form of the Latin Iohannes was Ean. In the northwest Spanish dialects and Portuguese the surname form of Ean was Eanes and Eannes. The es suffix functions like the Gaelic Mac. So Eannes means Son of Iohannes as does Mac Eáin. In modern Spanish, you see this surname written Yanes and Yáñes. It did occur to me there might have been some connection there, perhaps a loan word that had come into use from Portuguese or Spanish to Gaelic due to the increasing contact between Argyll and Spain during that time, but it would be a difficult point to research.
Mac Eóin itself is anglicised as MacEoin, McKeon, MacKeone, MacKeown, Keon, Keown, and MacOwen. It can be found from the southern tip of Ireland to the far north of Scotland, everywhere were Gaelic is and was spoken.
Seán came into Gaelic from the French form of the Latin Iohannes which was Jehan and Gaelicized as Seán. In the northern dialects of Gaelic, Seán is said 'Shane.' From this we get the surname Mac Seáin. Some of the descendants of the great Seaán Mór Ó Neill an Diomais use the surname MacShane today, while others have translated their surname and use both Jackson and Johnson. Andrew Jackson, the 7th president of the United Stages, claimed to be a direct descendant of Seaán Ó Neill and it is very true some of his descendants took the surname Jackson.
The Johnson and Johnston form are obvious as they are simple translations. Some families, for reasons unknown, chose Johnston and Johnstone, as the anglicised form of their Gaelic surname. It is a mistranslation as Johnston is from the two words 'John' and 'ton' the latter meaning a town.
It is very helpful know the Gaelic form of your surname and the anglicised forms also. Some of the non surname DNA matches may turn out to be just another form of your surname.