There's an exception for those that have naturalised but that would not be the case for anyone born in Northern Ireland. It's considered automatic.
This is a change to the previous rules
Irish citizen born in the South will be accepted as EU citizens in the United kingdom.
Irish citizens born in the North will not.
How can the Home Secretary justify splitting Irish citizens into two categories?
The change is that dual British nationals, British by birth, will not be considered an EEA national in the UK
The UKG has stated the people of Northern Ireland are "Automatically British by birth"
Evidenced by my own court case
I'm an Irish national being denied their EU rights & told they're British
This policy appears to be expanding
“UK Government Refuses to recognise Irish citizenship” link.medium.com/7uQRQL2IvV