Ranko Matasović points out that there are words of possibly or probably non-Indo-European origin in other Celtic languages as well; therefore, the substrate may not have been in contact with Primitive Irish but rather with Proto-Celtic.
Ranko Matasović has provided a list of changes which affected both branches of Insular Celtic but for which there is no evidence that they should be dated to a putative Proto-Insular Celtic period.
He contributes to the project of Indo-European Etymological Dictionary organized by the Leiden University by making An etymological lexicon of Proto-Celtic.