How old are the earliest surviving Irish-language inscriptions written in ogham?

The earliest surviving Irish-language inscriptions in ogham are generally dated to the 4th century CE, with some scholarly estimates extending into the 5th century CE.12

These inscriptions, which consist of an alphabetic script composed of strokes and notches, represent the earliest evidence of the Irish language. They are typically found on stone monuments and usually contain a name and patronymic in the genitive case. While some sources associate the script with the 5th and 6th centuries, the consensus among specialists is that the tradition of these archaic sepulchral inscriptions began as early as the 4th century.123