Celtiberian (also Hispano-Celtic) is an extinct Celtic language spoken by the Celtiberians in Portugal and central Spain before and during the Roman Empire. Very little remains of the Celtiberian language, which is attested in some pre-Roman placenames in the Iberian peninsula that survived long enough to be recorded in documents, in the formulas that were used in some personal names (giving hints of grammar), and in some inscriptions on bronze and lead plaques, written in the Celtiberian script that combines Phoenician and Greek characteristics. Enough has been preserved to show that the Celtiberian language was Q-Celtic (like Goidelic), and not P-Celtic like Gaulish (Mallory 1989, p. 106). This seems to confirm at least some of the legends preserved in the Leabhar Gabhala, which state that the first descendants of the Irish people arrived from Iberia.
Since Brythonic is P-Celtic too, but as an Insular Celtic language more closely related to Goidelic than to Gaulish (q) to p also occurred in some Italic languages: compare Oscan pis, pid ("who, what?") with Latin quis, quid. Celtiberian and Gaulish are usually grouped together as the Continental Celtic languages, but this grouping too is paraphyletic: no evidence suggests the two shared any common innovation separately from Insular Celtic.
The longest extant Celtiberian inscriptions are those on three Botorrita plaques, bronze plaques from Botorrita near Saragossa, dating to the early 1st century BC, labelled Botorrita I, III and IV (Botorrita II is in the Latin language).
Celtiberian exhibits a fully inflected relative pronoun ios, not preserved in other Celtic dialects, and the particles kue "and", nekue "nor", ve "or". Like in Welsh, there is an s-subjunctive, gabiseti "he shall take" (Old Irish gabid), robiseti, auseti. Compare Umbrian ferest "he shall make".

Sources

Jordán Cólera, C. (2005). Celtibérico. Zaragoza.
Hoz, Javier de. (1996). The Botorrita first text. Its epigraphical background; in: Die größeren altkeltischen Sprachdenkmäler. Akten des Kolloquiums Innsbruck 29. April - 3. Mai 1993, ed. W. Meid and P. Anreiter, 124–145, Innsbruck.
Mallory, J. P. (1989). In Search of the Indo-Europeans. Thames & Hudson. ISBN 0-500-05052-X
Meid, Wolfgang. (1994). Celtiberian Inscriptions, Archaeolingua, edd. S. Bökönyi and W. Meid, Series Minor, 5, 12–13. Budapest.