Indo-European, Celtic, and Irish

Linguists believe that almost all the languages spoken between Iceland and Bangladesh are related. Their common ancestor, known as Indo-European, would have been spoken around six thousand years ago, either on the Russian steppe north of the Black Sea and the Caspian Sea (the prevailing hypothesis) or in Anatolia (the leading alternative view).
Whoever spoke Indo-European didn't write any of it down, and so whatever linguists claim to know of it, they know only indirectly by comparing its descendants. It's fairly obvious that modern French, Spanish, Italian, Catalan, Romanian and so forth are all related, and by comparing them with each other we could reconstruct a common ancestral language for them. In fact, we know that they developed in historical times from Latin. Similarly, English, German, Dutch, Flemish, Swiss German, and so forth are all obviously related, and developed from a common ancestor in the last couple of thousand years. There are lots of groups of languages like this, including the Slavic group, the Celtic group, and the Indic group. Indo-European would be the common ancestor of all these groups, the great-great-great-grandmother of all these related languages.

The Celtic group spread out all over continental Europe a couple thousand years ago. When Paul wrote his letter to the Galatians, he was writing to a Celtic-speaking community in what is now Turkey. The Gauls with whom Caesar fought his famous war in what is now France were a Celtic-speaking people, as were at least some of the people who inhabited modern Spain and Portugal in Caesar's time. Place names from "London" to "the Danube" are all of Celtic origin. But the islands of Ireland and Great Britain are the only places whose Celtic languages survived into modern times. The dialects from Ireland are called Goidelic (as in "Gaelic"), while those from Britain are called Brythonic (as in "British"). Linguists talk about "Insular Celtic", in the belief that Goidelic and Brythonic were more closely related to each other than either was to continental dialects, although there isn't complete agreement on that point.

Goidelic gave rise to three modern languages, Irish Gaelic, Scottish Gaelic, and Manx Gaelic. Manx became extinct in this century, at least in so far as there is no longer a community for whom it is the normal spoken language, although there are quite a few fluent Manx speakers around and it is the centre of a lively revival. Scottish Gaelic had its origin in Gaelic-speaking tribes from Ireland who invaded what was then Pictland round about the fifth century AD. They spread out and established the Gaelic kingdom of Scotland. The language is spoken today in the Outer Hebrides islands and in some places in the Highlands. Back in Ireland, there are a few communities, mostly on the west coast, where Irish is the primary spoken language. It is the first official language of the Republic of Ireland and is a compulsory subject in primary school there, while in the Six Counties of Northern Ireland, Irish has recently been granted official status as part of the Good Friday Agreement.

Brythonic also gave rise to three modern languages, Welsh, Cornish and Breton. Cornish became extinct at the end of the eighteenth century, although, like Manx, it is the centre of a revival movement. Breton is spoken in Brittany in France, although it is not descended from Gaulish: its first speakers migrated from Britain in the early part of the first millenium, pushed out by the invading Germanic tribes who later became the English. Welsh is probably the Celtic language in the strongest position today, with over half a million speakers and extensive institutional support in the spheres of religion, local government, education and broadcast media. Although there are probably many more speakers of Breton, they are almost all illiterate in their own language and are bereft of institutional support.

The Goidelic languages are often called Q-Celtic, and the Brythonic (and continental) languages are called P-Celtic. The Q and the P refer to a certain sound, originally something like qu- in Indo-European, which became p- in the Brythonic languages but remained qu- in the Goidelic ones, eventually becoming c-. The same sound remained a qu- in Latin, but became wh- or f- in English, as seen in the following chart. (Strictly speaking, the qu- sound became wh- in English but was replaced in some words with a p-, which later became an f-.)
 

The correspondence between Irish c- and Welsh p- must have been well-known to speakers on both sides of the Irish Sea in the fourth and fifth centuries, when Ireland was being evangelised from Wales. Many words borrowed from Latin show Welsh influence, and in some words, the original p- was replaced with a c-, along the lines of the inherited pairs shown above. Thus the Latin word Pascha "Easter" was borrowed into Irish as Cáisc; the Latin word planta "shoot, plant" was borrowed as clann, meaning "family".