There are many examples of 2 things appearing in 2 different parts of Europe. For example, Galicia in Spain and Galicia in Poland and Ukraine, although the latter is just a Latinized form of another word.
When it comes to similarities, yes, Celts used to live in various parts of Europe.
And then the big-ass Roman republic and empire. They built roads across Europe.
It was easier for whole nations/tribes to travel.
Please, write down some examples of those toponyms, hydronyms, the grammatical constructs and so on that are similar between South Slavic and (old) Irish. Thanks.