i think both are slavic, and my last argument for this is the way the english treat both.
What, you mean allow them to come into England and work on building sites for a minimum wage?
there is as much Saxon DNA in modern Irish as Celt.
and probably more than a little Norman, too.
Being underdogs and having had a rough history,
I know what you mean, but it's worth considering that the Poles were pretty much top dogs in their area for a loooong time before 18th C. Granted, history made up for a lot of that in the next 2 centuries.
The Polish also over-identified with the church, due to Godless communism.
Not sure about that. I think it's even earlier than that. Prior to the carve-up of Poland in laate 18th C it was a very multi-ethnic/ multi-religious state. Arguably following the carve-up two of the main powers were Protestant and Orthodox Christians, and, certainly on the Russian side, there is supposed to have been a lot of anti-catholic activity. The ex-pat writers, such as Mickiewicz, who created a "lost Polishness" (in a similar way that Sir Walter Scott created a "lost Highland" culture in Scotland) tended to portray "Polishness" as catholic (something Sienkiewicz continued with some of his works).
Is any catholic country really much different?
Well, Poland tried with uniforms and messed it up totally. It was such a fiasco there should have been a national enquiry into it.