Terminology
The modern English term Gael derives ultimately from the Old Irish (Ancient Gaelic) word Goídel, which was spelled in various ways by Gaelic writers at different times. The modern Gaelic spellings are Gael (Irish) and Gàidheal (Scottish Gaelic).
Early Greek and Roman authors called the Irish Ιουερνοι and Iverni, respectively, both derived from the Proto-Irish ethnic name *Iwerni ("people of *Iweriū"). Later Greek and Latin variants of this name included Ίερνοι, Hierni, and Hiberni.
Scoti or Scotti was another generic Latin name for the Irish that came into use by the 4th century. It is not believed that any Gaelic groups called themselves Scot(t)i in ancient times, except when referring to themselves in Latin. It is also conjectured that the Latin term may mean "raider/pirate" as it is widely accepted that raiders from Ireland were attacking Britain's west coast during and following the Roman occupation.
Goidel is thought to have been borrowed some time during the 7th century from the (not directly attested) Primitive Welsh form which became Old Welsh Guoidel, i.e. "Irishman" (which is attested as a male personal name in the Book of Llandaff), and may ultimately be derived from the Proto-Indo-European *weidh-(e)l-o-, perhaps meaning "forest people", partially cognate with the Old Irish ethnic name Féni (from Proto-Indo-European *weidh-n-jo-, "forest people", later becoming simply "warriors" in Proto-Irish).
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