Which language is written in the unique script known as Ogham in its earliest inscriptions?

Primitive Irish is the language written in the earliest Ogham inscriptions.

Primitive Irish was an early form of the Irish language recorded mainly on stone monuments from the fourth to sixth centuries. These inscriptions use Ogham, a system of lines and notches arranged along the edge of a stone or another surface. The writing is especially associated with Ireland and parts of western Britain.

Most surviving Ogham inscriptions are short, commonly recording personal names, family relationships, or ownership. They are important evidence for the earliest stage of Irish and for the development of the Goidelic branch of Celtic. Later Irish manuscripts used the Latin alphabet rather than Ogham.

Ogham is sometimes described as a Celtic alphabet, but it is more precise to call it an early inscriptional writing system used primarily for Primitive Irish. A small number of inscriptions occur in other languages or are uncertain, so the strongest association is with early Irish rather than with every Celtic language.