Never, under any circumstances, mix pantheons. It is more than fine to worship deities of different pantheons, but maintaining cultural relevance in regards to the mythologies and pantheons is key. It is disrespectful to mix worship and deities when their origins are vastly different.
I think it depends on what you mean by ‘mix pantheons’, and the answer would partially depend on the rules of each pantheon/deity. Many people have multiple practices that they work side by side (sometimes called multi-trad) and it works alright. Its just that you wouldn’t want to shove, say… an Egyptian deity into a Shinto kamidana. Mixing in that respect is a no-go. But if you’ve got a separate shrine for your Dutch deities, and another shrine for Irish deities (to use your examples, anon), then you should be alright- provided those gods have given the go ahead.
But it does depend on what you mean by ‘mix pantheons’. There are definitely more respectful ways and less respectful ways to do that.