Irish Gaelic: Tomas Mac Sheain
Irish Gaelic, or Gaeilge, renders Thomas as Tomas, written with a fada (the long-vowel accent mark) over the o. Pronounced TOH-mahs, the name has been in use in Ireland since the early medieval period, primarily through Christian tradition.
The surname Johnson presents a more interesting case. Irish surnames traditionally used the Mac prefix (meaning son of) or O prefix (meaning grandson or descendant of). The closest Irish equivalent to Johnson would be Mac Sheain, son of John, where Sean is the Irish form of John. The full name Tomas Mac Sheain reads as Thomas son of John in formal Irish, though most modern Irish speakers named Thomas Johnson would simply use the English form in writing.
According to records maintained by the Government of Ireland, Sean and its variants remain among the most popular boys' names in Ireland. The patronymic structure built on Sean produced surnames including Mac Sheain, anglicized in some cases to McShane, and the Irish census shows these surnames concentrated in Ulster.
Scottish Gaelic: Tomas MacIain
Scottish Gaelic, called Gaidhlig by its speakers, sits close to Irish but maintains its own conventions. Thomas appears as Tomas, sometimes spelled Tamhas or Tavish in older sources. The pronunciation in Gaidhlig leans toward TAH-vahsh in some dialects, particularly in the Western Isles.
The Scottish patronymic for John is MacIain, written Mac Iain, meaning son of John (Iain being the Gaelic form). The surname MacIain anglicized into MacEan, McKean, and eventually contributed to the surname Johnson in lowland Scotland, where Scots speakers often shifted patronymics into the English style during the 17th and 18th centuries.
The BBC's Gaidhlig service and academic resources from the University of Edinburgh document this shift, noting that many Scottish Johnsons today have ancestors who once carried Mac Iain or one of its variants. A Tomas MacIain in 16th-century Argyll could easily have been a Thomas Johnson by the time his great-great-grandchildren emigrated to North America.
Welsh: Twm ap Ieuan
Welsh handles names through a system that feels almost playful to outsiders. Formal Welsh uses Tomas, but the everyday short form is Twm (pronounced approximately like the English Tom). Welsh speakers historically built patronymics with ap, meaning son of, attached to the father's first name.
John in Welsh is Ieuan or Ifan, both old forms of the same Hebrew name. A full Welsh patronymic for Thomas Johnson would be Twm ap Ieuan or Tomas ap Ifan. Over time, the ap-construction collapsed into surnames. Ap Ieuan became Bevan or Beavan. Ap Ifan became Evans. So a modern Welsh Thomas Johnson, traced through the patronymic system, would more naturally read as Twm Evans or Tomas Bevan.
The shift happened gradually between the 16th and 18th centuries, accelerated by English administrative pressure to standardize surnames. Resources at the National Library of Wales preserve parish records showing this transition in real time, with the same family appearing as ap Ieuan in one decade and Bevan in the next.
Cornish, Manx, and Breton
The three remaining Celtic languages each retain their own version of the name, though with smaller speaker communities.
Cornish, revived from near-extinction in the 20th century, uses Tommas or Tomas, with patronymic constructions similar to Welsh. The Cornish for John is Yowann, and son of would build into Mab Yowann in formal usage, though this is rarely encountered in everyday modern Cornish.
Manx Gaelic, spoken historically on the Isle of Man, uses Thomase or Tommee. The Manx for John is Juan, and the patronymic Mac Juan appears in 18th-century records, often anglicized as Maguan or eventually losing the patronymic structure entirely as Manx surnames stabilized.
Breton, spoken in Brittany on the French Atlantic coast, uses Tomaz. John in Breton is Yann or Yannig, both familiar forms. The Breton patronymic for son of John is Mab Yann, though Breton surnames more often took place-based or descriptive forms. A modern Breton named Thomas Johnson would likely just keep the English form, particularly given Brittany's location within France and the dominance of French in official records.
These three languages collectively have fewer than 100,000 speakers, according to surveys cited by UNESCO and the European Charter for Regional or Minority Languages. The forms above are linguistically accurate but rarely encountered in practice.
Why the Celtic Forms Look So Different
If you compare Tomas Mac Sheain or Twm ap Ieuan to the English Thomas Johnson, the surface differences are dramatic. The roots, however, are the same.
Thomas crosses into all six Celtic languages through Latin and the spread of Christianity. The Aramaic original meaning twin survives intact in each language, even when the spelling changes. Johnson, despite looking distinctly English, descends from the same Hebrew root (Yochanan, meaning God is gracious) that produced Sean, Iain, Ieuan, Ifan, Yowann, Juan, and Yann across the Celtic languages. The patronymic structure (Mac, ap, Mab) is a Celtic feature that the English Johnson construction (using the suffix -son) parallels rather than borrows.
What you are looking at when you read these variants is the same name expressed in linguistic systems that diverged from a common Indo-European ancestor several thousand years ago. The Celtic languages just preserve some of the older patterns more visibly than English does, and they each developed their own rules for how to talk about who someone's father was.