As a unionist, I'm not particularly in favour of ROI essentially treating northern Ireland as part of their territory by granting people born in NI ROI passports. I also think that to be able to claim citizenship through grandparents is incredibly lax - most other countries limit it to parents and then in many cases it isn't automatic.
However, on an individual level if you're eligible to apply for an Irish passport (due to their extremely lax laws) and maintain EU citizenship post brexit then I absolutely understand why people would do this. (I do think it's a bit rich for brexiteers like Iain paisley jnr to do it!) I also understand why people might think it's a bit cheeky - we were fortunate to secure a second EU passport for dd (Not irish). We have no real links to the country, speak the language very badly and don't plan to live there. But under their laws they considered dd a citizen of their country since the moment she was born, passport or no passport.