If it has any relation to Celtic languages, then it's Indo-European by definition.

We can tell how much neanderthal ancestry someone has, more or less. Basque people have no more than others. Despite their odd language, they are much like other Europeans genetically: a similar mix of European hunter gatherers, Anatolian farmers and the bronze age invaders which we believe brought the IE languages to Europe.

Oh! Sorry- I meant to refer to the hypothetical proto-Celtic language!

This is what vibe-commenting from memory gets me.