How do I know how strong my Irish roots are?

I have Irish hertiage from my dad’s side(we still aren’t sure who was all Irish yet but we are looking into it) and a lot of family members on his side have blonde/blond hair and green/hazel eyes and some have red hair. My last name is Irish and my nephew even has red hair and green/hazel eyes when his parents have dark hair and dark eyes(just me my sister didn’t cheat). We want to take the test but it is expensive.