When discussing the concept of languages which may have formed a substrate to the Indo-European, Ireland is an interesting case.

An isolated island on the periphery of European folk movements, it may have preserved evidence of the earliest linguistic phases free from linguistic imports from the Mediterranean. The island has been inhabited from the Paleolithic and it is certain that many thousands of years of linguistic development preceded the introduction or the transmission of the Indo-European Goidelic variety of Celtic.

Early forms
The Masilote Periplus gives the name "ierne" (compare Old Irish "eriu") and Latin authors give Hibernia. Ptolemy gives the name of the island and a sparse list of names of localities and natural features. Some of these are consistent with the presence of a Goidelic speaking population. Others defy analysis.

Old Irish
As recorded in the fourth century Old Irish, is a very primitive language following closely the structures of Sanskrit and obviously just emerging from the archaic stage. This brings it close to the hypothetical Common Celtic and argues for an early date for a Celtic speaking population in Ireland.

What preceded this early Celtic?
The answer is linked to the chronology for the introduction or the adoption of an Indo-European derivative language in Ireland. The earliest date seriously proposed falls in the Irish Bronze Age (1800 B.C.E), the latest within the Irish Iron Age.

An early date would imply that an early form of the Celtic language family was spoken in Ireland in the Bronze Age and that it developed in situ to the Old Irish first attested in 300 C.E. The later date allows for the importation or the adoption of Old Irish in the fourth or the third centuries B.C.E.

This late date would necessitate the mutation of Q-Celtic to P-Celtic in Great Britain immediately prior to the historical period if Britain is postulated as the source for Old Irish. If not, the only other source is Iberia and this would imply that a language similar to Old Irish was spoken in the peninsula in the early centuries B.C.E. There is no evidence for the presence of such a language.