Living in Ireland, great-grandmother born in Ireland… Is it worth trying by association?

Hi there. Posting for a friend, so I’ll be brief.

My friend’s son (I’ll call him R) has been living and working in Dublin for three years. Big international corporation, so proper pay slips, PAYE, and all the rest.

R’s great-grandparents were born in Cork, but R’s grandmother and mother were born in the UK, as was he.

R’s grandmother and mother eventually got Irish citizenship but not until after R’s birth. So he can’t benefit from the FBR.

R’s grandmother retired to Ireland years ago so Ireland is now the place for family gatherings.

My instinct is that there’s enough of a connection for him to try for naturalisation by association. But I’m no expert. And I don’t want to encourage R if there’s really little chance.

Would anyone be willing to offer some advice? Just pouring cold water on the idea is fine too!

Thank you!