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Religion wasn't the problem. It just got dumbed down and diluted into religion issue because it is simpler for people.

Back when people were fighting for Irish unity, protestant and catholics were fighting together on the same side. The division comes from the historical and recent colonialism, racism, genocide, and injustices carried out against the people of Ireland and the ensuing aftermath and retaliations. Hatred from this has been passed down the generations and has become a catholic versus protestant thing. It's become a very tribalistic mindset. People vote based on religion and who their parents voted for. Hatred is passed down from father to son.

We were always one country over the years, but with different Kings and Queens across different regions.