There was a unifying language spoken by the Celts, called not suprisingly, old Celtic. Philogists have shown the descendence of Celtic from the original Ur -language and from the Indo-European language tradition. In fact, the form of old Celtic was the closest cousin to Italic , the precursor of Latin.
The original wave of Celtic immigrants to the British Isles are called the q-Celts and spoke Goidelic . It is not known exactly when this immigration occurred but it may be placed somtime in the window of 2000 to 1200 BC. The label q-Celtic stems from the differences between this early Celtic tounge and Italic . Some of the differences between Italic and Celtic included that lack of a p in Celtic and an a in place of an the Italic o .
At a later date, a second wave of immigrants took to the British Isles, a wave of Celts referred to as the p-Celts speaking Brythonic . Goidelic led to the formation of the three Gaelic languages spoken in Ireland, Man and later Scotland. Brythonic gave rise to two British Isles languages, Welsh and Cornish, as well as surviving on the Continent in the form of Breton, spoken in Brittany.
The label q-Celtic stems from the differences between this early Celtic tounge and the latter formed p-Celtic . The differences between the two Celtic branches are simple in theoretical form. Take for example the word ekvos in Indo-European , meaning horse . In q-Celtic this was rendered as equos while in p-Celtic it became epos , the q sound being replaced with a p sound. Another example is the Latin qui who . In q-Celtic this rendered as cia while in p-Celtic it rendered as pwy . It should also be noted that there are still words common to the two Celtic subgroups.
As an aside, take note that when the Irish expansion into Pictish Britain occurred (see below), several colonies were established in present day Wales. The local inhabitants called the Irish arrivals gwyddel savages from which comes geídil and goidel and thus the Goidelic tounge.