There are a great many words in Irish Gaelic that seem to have a stronger etymological connection to Slavic (and in particular South Slavic) word than they do with other Indo-European languages (many other such links are noted at the same blog). There are archaeological and oral history suggestions of these links as well.
Conventional linguistic family trees don't suggest such a connection. But, there is almost surely an important lesson about Western European pre-history hidden in there somewhere, but a really articulate story, providing either a direction of transmission, or better yet, a historical narrative of this connection is lacking.
My intuition is that the pre-Slavic language of the region may have been pre-proto-Celtic, that a significant migration from this region may have reached its terminus in Ireland, and that this substrate may have influenced the adoption of the Slavic family of languages of the region. Notably, frontier regions often end up having more pure transmission of the source language and culture of their founding population that intermediate ones, which would help explain why Irish Gaelic might have more pure connections to South Slavic than, for example, Welsh.