Celtiberian is an extinct Celtic language once spoken on the Iberian peninsula until about the 1st century BC, particularly between the Duero, Tajo, Júcar, Turia and Ebro rivers in the north of what is now Spain.
Celtiberian is thought to have been a Q-Celtic language related to the Gaelic/Goidelic languages of Ireland, Scotland and the Isle of Man. There are Old Irish legends preserved in the Lebor Gabála Érenn (The Book of Invasions) name the Milesians, a people from Iberia, as being the ancestors of the Irish. The Celtiberians themselves are thought the have migrated to Iberia from Gaul (France), perhaps during the 6th century BC.
The name Celtiberi appears in the writing of Roman authors such as Diodorus Siculus, Appian and Martial, who thought these people were a mixture of Celts (Celtae) and Iberians (Iberi), with the Celts being dominant. They also recording the tribal names Arevaci, Belli, Titti and Lusones.
The Celtiberian script developed from the Iberian scripts. About 200 inscriptions in Celtiberian have been found in the Celtiberian alphabet, and also in the Latin alphabet, dating from between the 6th and 1st centuries BC. With the Roman take over of the Iberian peninsula, the Celtiberian script was gradually replaced by the Roman/Latin alphabet and eventually disappeared.
From Uxama (Western script) [source]
Information about Celtiberian
Iberian Epigraphy Page, by Jesús Rodríguez Ramos - details of the scripts and languages of pre-Roman Iberia (Spain and Portugual): http://www.webpersonal.net/jrr/
ALPHABETUM - a Unicode font specifically designed for ancient scripts, including classical & medieval Latin, ancient Greek, Etruscan, Oscan, Umbrian, Faliscan, Messapic, Picene, Iberian, Celtiberian, Gothic, Runic, Old & Middle English, Hebrew, Sanskrit, Old Nordic, Ogham, Kharosthi, Glagolitic, Old Cyrillic, Phoenician, Avestan, Ugaritic, Linear B, Anatolian scripts, Coptic, Cypriot, Brahmi, Old Persian cuneiform: http://guindo.pntic.mec.es/~jmag0042/alphabet.html