The question of the arrival of Celtic-speaking peoples in Britain and Ireland, i.e. the Celticisation of these islands in prehistory, has been a bone of contention – and a mystery – since the early days of Celtic Studies. For the longest time, the only two disciplines that could meaningfully contribute to this debate were archaeology and historical linguistics. However, during the latter part of the 20th century the realisation grew that the methodologies and the data of the two disciplines are not commensurate, and that the conclusions drawn by scholars from the involved disciplines are not compatible with each other. Since 2015, a third discipline with an entirely new type of data has joined the discussion: palaeogenetics. The analysis of ancient DNA provides a completely different perspective on the prehistory of the Western Archipelago and allows insights into it that neither archaeology nor linguistics are able to achieve with their own methods, and that can inspire the two traditional disciplines to look at their data again and reevaluate it. This talk is a modest attempt from the point of view of historical Celtic and Indo-European linguistics to respond to the new insights and challenges that palaeogenetics has to offer, and to assess how they can be integrated into a general theory of the Celticisation of the Western Archipelago.