I didn't really get the Irish fans' problem with Rice and Grealish. Neither was born in Ireland and both seem to have lived their entire lives in England.
Irish people don't have an issue with players choosing to play for their nation of birth over their ancestry eg Kane, Maguire, Gallagher and Bellingham all could have played for Ireland but played for England. I have never heard an Irish person criticise them for that choice.
Grealish and especially Rice is a different scenario. Both played underage for Ireland and eventually chose to play for England, which they were perfectly entitled to. Michael Keane did the same thing and no-one has an issue with this. I think the issue with Grealish is that he is Grealish. Remember how we all felt about him during the "hand of Rod" game. He spent half the game dropping on his backside if someone so much as sneezed in his direction. He is a player opposition fans really dislike, if you notice in yesterday's match he didn't get booed every time he got the ball like Rice did.
The Rice situation is different and more complex. He played friendlies for the senior team, expressed his commitment to Ireland, kissed the badge etc. Then proceeded to flip and flop over which team he would represent before evenually picking England. This also took a space in the international set-up from a player who could have represented Ireland. Wasting FAI resources who have a considerably smaller budget than the FA. How would you feel if a player chose another country having represented England at senior level?
This is before you take into account the history between Ireland and England. How would you feel about a player who chose to play for the country who oppressed your country for 800 years? To choose the country who singlehandedly were responsible for halving the population during the great hunger, which lead to the death of over a million people and the mass emigration of millions of others escaping starvation. While Irish and English people get on with one another on an individual basis, a nation's psyche is affected by it's history and runs deep in all nationalities and is very difficult to ignore. While it is only football, it is rarely ever only football.