There is little doubt that the early stages of the subgroups of the Indo-European language family involved extensive contact. The movements of early groups of speakers across large stretches of land in Euroasia meant that these people came into contact with others who spoke genetically unrelated languages. This contact is responsible for the non Indo-European lexis in Indo-European languages and may also be the source of non-inherited grammatical features. Establishing the precise source of such lexis and grammar is a daunting task, given the great time-depth involved and the dearth of textual records that could provide helpful data for reconstructing the sources of borrowings external to this language family. But there was also contact within the orbit of the Indo-European languages when members of different subgroups came into close geographical proximity with each other due to repeated migrations. This fact accounts for borrowings across Indo-European subgroups (e.g. from Celtic into Germanic). This chapter examines cases of contact and probable borrowing both within the Indo-European language family and at its external interface to languages from other families, inasmuch as this can be established with reasonable certainty. The focus for this treatment is on early stages both of Celtic and of the Irish language as one of the main members of this group. The consideration of contact effects in Irish is limited to the language as it developed up to the late Middle Ages.