Enough has been preserved to show that the Celtiberian language could be called Q-Celtic (like Goidelic), and not P-Celtic like Gaulish. For some, this has served to confirm that the legendary invasion of Ireland by the Milesians, preserved in the Lebor Gabála Érenn, actually happened.
Some scholars believe that Brythonic is more closely related to Goidelic than to Gaulish; it would follow that the P/Q division is polyphyletic, the change from kʷ to p occurring in Brythonic and Gaulish at a time when they were already separate languages, rather than constituting a division that marked a separate branch in the "family tree" of the Celtic languages. A change from PIE kʷ (q) to p also occurred in some Italic languages and Ancient Greek dialects: compare Oscan pis, pid ("who, what?") with Latin quis, quid; or Gaulish epos ("horse") and Attic Greek ἵππος hippos with Latin equus and Mycenaean Greek i-qo. Celtiberian and Gaulish are usually grouped together as the Continental Celtic languages, but this grouping is paraphyletic too: no evidence suggests the two shared any common innovation separately from Insular Celtic.
Celtiberian exhibits a fully inflected relative pronoun ios (as does, e.g., Ancient Greek), not preserved in other Celtic languages, and the particles -kue 'and' < *kʷe (cf. Latin -que, Attic Greek τε te), nekue 'nor' < *ne-kʷe (cf. Latin neque), ekue 'also, as well' < *h₂et(i)-kʷe (cf. Lat. atque, Gaulish ate, OIr. aith 'again'), ve "or" (cf. Latin enclitic -ve and Attic Greek ἤ ē < Proto-Greek *ē-we). As in Welsh, there is an s-subjunctive, gabiseti "he shall take" (Old Irish gabid), robiseti, auseti. Compare Umbrian ferest "he/she/it shall make" or Ancient Greek δείξῃ deiksēi (aorist subj.) / δείξει deiksei (future ind.) "(that) he/she/it shall show".