Our first attestations of a Celtic language hail from the Swiss-Italian border and date from the 6th century B.C. These early Lepontic inscriptions were written in an Etruscan-derived script and survive primarily on coins and on stone as commemorative epitaphs. Lepontic seems to have been the language of Celts who dwelt in Cisalpine Gaul, the Roman name for lands south of the Alps but north of the Rubicon River. The Cisalpine Celts made deep incursions on the Italian peninsula in the early 4th century B.C. that culminated in the sack of Rome; but the ascendant Romans rallied to conquer the region by the 2nd century B.C., and Cisalpine Gaul was reckoned a Roman territory thereafter. The last inscriptions in Lepontic date from around this time, suggesting that Latin quickly established itself as the dominant language of the region while Lepontic drifted toward silent extinction.
Across the Alps lay the vast lands of Transalpine Gaul, dominated by Celtic tribes until the last century B.C. The earliest records of their Gaulish language survive from the late 3rd century B.C. on stone monuments and potsherds, written in an adaptation of the Greek alphabet borrowed from Greek merchant-colonists of Marseilles. Further inscriptions on lead and bronze tablets yield precious information about early Celtic culture and religion. In the decades following the Gallic Wars of the mid-1st century B.C., which resulted in the Roman conquest of Gaul (the subject of Caesar's De Bello Gallico), attestation of Gaulish breaks off. Just as Latin had marginalized Lepontic in Cisalpine Gaul, it came to oust Gaulish as the primary language of Transalpine Gaul. Yet spoken Gaulish evidently died a slow death, for we find a Gaulish-language calendar dating to the Christian era, and medieval sources indicate that residual use of Gaulish may have continued into Late Antiquity.
A century after Lepontic-speaking Celts ravaged Rome, speakers of another Celtic language converged on the Anatolian peninsula and there established a thriving community that survived from its establishment in the 3rd century B.C. to at least the days of Paul the Apostle. The Romans named the province Galatia. As the Galatian language is only attested in toponyms, in personal names, and on coins, there is insufficient evidence to follow scholars who classify it as a Gaulish dialect.
Still other Celtic tribes had invaded and settled the Iberian Peninsula in the remote west of the Mediterranean as early as 500 B.C. Several non-Indo-European languages (including one ancestral to Basque) were spoken in Iberia prior to the arrival of Celts; from one such language, Iberian Celts adopted a syllabic script to record their own. Celtiberian is attested on coins and bronze tablets dating to the first century B.C., having survived the Roman annexation of Iberia in the previous century. Nevertheless, the increasing Romanization of Iberia ensured that Latin finally marginalized this Celtic language as well as the others spoken on the mainland.
The Celtic languages in the foregoing discussion are often grouped in a Continental phylum of Celtic languages, to distinguish them from the Insular Celtic languages of the British Isles. While the attested languages of Insular Celtic feature linguistic innovations common and exclusive to that group, no parallel innovative unity is found among the Continental tongues. It is accordingly best to view the "Insular" designation as a genetic bond, whereas "Continental" is merely a geographic term (also implying "not Insular").
Julius Caesar's abortive invasion of England in the mid-1st century B.C. left the enterprise of Roman conquest unfulfilled there for several generations, until successive campaigns under the later Julio-Claudians at last established Roman control of the island's southern two-thirds by the end of the first century A.D. Rome won this large swath of territory primarily at the expense of Celtic tribes which had crossed the English Channel some thousand years before (though dates for Celtic migrations to the British Isles are quite uncertain). In the end, Roman conquest brought about several hundred years of fruitful cultural interaction, where Celts living under Roman authority came to know such Mediterranean insitutions as theatres and bath-houses, while in exchange Celtic cults found their way into Roman temples. Latin itself came to exert a moderate, mostly lexical influence on the Celtic dialects then spoken on the island, most or all of which were early members of the Brythonic phylum of Insular Celtic. The mounting cost of controlling the outer provinces, in light of the Empire's late 4th century fiscal problems, forced a Roman withdrawal from Britain by the early 5th century. The subsequent unrest that befell southern Britain gave Germanic Angles and Saxons from the North Sea coast of Europe an opportunity to migrate there, settling there in large numbers over the following decades. By the mid-5th century, Germanic tribes had won several pitched battles against the Brittonic Celts and laid the foundation for Anglo-Saxon cultural and linguistic dominance -- and, as well, created a pretext for the Celtic Tales of King Arthur, whose name is mentioned in the mid-7th century Welsh poem Y Goddodin as if he were already the subject of legend.
The successive and enduring conquests which had caused Brythonic languages to lose speakers and territory -- first to Latin, then to dialects of Anglo-Saxon -- nevertheless proved unable to eradicate them. A group of Brythonic speakers left Britain following the Anglo-Saxon conquest and established new homes in Brittany (northwest France), where their Breton language, first attested in the 8th century, survives to this day. Breton has the unfortunate distinction of being the only Celtic language with substantial land gains since the Gallic Wars. A closely related language is Cornish, which tentatively first appears in the marginalia of a 9th century copy of De Consolatio Philosophiae; it clung to Britain's southwest up through the 18th century. And there are today half a million speakers of the Brythonic dialect Welsh, spoken in Wales and attested in archaic forms (e.g. in the poem mentioned above) from the 6th or 7th century A.D.
Old Irish is by far the most copiously attested Celtic language of premodern times. The first Old Irish texts have been preserved from the 5th century A.D. and comprise a great body of poetry, saga, and religious literature. The written language was much standardized and supplemented Latin as a common literary tongue between Ireland, Britain and the Continent from the 7th century. By the 10th century, accumulated linguistic change reflected in the written standard had come to distinguish a descendant language, Middle Irish. This in turn gave rise to today's Irish Gaelic of Eire; as well, migration and prolonged settlement in the centuries after A.D. 1000 saw Middle Irish-speaking communities in Scotland and on the Isle of Man develop the languages Scottish Gaelic and Manx, respectively.