Short answers, pulled from the story.
The term Celtic first appeared in print when Edward Lhuyd used it to describe a specific group of languages in 1707. Paul-Yves Pezron had previously linked the Celts described by classical writers with Welsh and Breton before Lhuyd formalized the connection.
Lepontic stands as the oldest attested Celtic language dating back to the sixth century BC. Anciently spoken in Switzerland and Northern-Central Italy, it left behind coins inscribed in Noricum and Gallia Narbonensis.
Irish has between 40,000 and 80,000 native speakers according to SIL Ethnologue estimates. In the Republic of Ireland, 73,803 people use Irish daily outside the education system as of recent counts.
Cornish and Manx languages died out in modern times with their last native speakers passing away in 1777 and 1974 respectively. Revitalisation movements during the 2000s led to the re-emergence of native speakers for both languages.
By the fourth century AD, Latin script had replaced older writing systems for all Celtic tongues. Early Continental inscriptions utilized Italic and Paleohispanic scripts before the shift to Latin occurred.