Sorry, I was talking about citizenship not birth certs really. There is a difference. You are a British national if born in NI in most cases, not an Irish one ( though entitled to citizenship it's not automatic).
This is what Ireland's (ROI's) Citizen's Information site says about Irish citizenship.
"1. Born in Ireland before 1 January 2005
If you were born in Ireland before 1 January 2005, you are an Irish citizen by birth.
If you were born in Northern Ireland before 1 January 2005, you are entitled to claim Irish citizenship. This means that you can choose to be an Irish citizen and apply for an Irish passport if you want to.
2. Born in Ireland after 31 December 2004
...
If either of your parents was an Irish or UK citizen at the time of your birth, you are automatically an Irish citizen if you were born in Ireland. If you were born in Northern Ireland to an Irish or British parent, you can choose to be an Irish citizen."
Thus, if you are born in NI you are automatically a British citizen by birth mostly , even if you do not choose to be and never have a British passport. Renouncing a British passport - which is neccessary for some immigration cases if you want to be considered Irish - costs you time and money. Hence De Souza's objection. It's not the same the other way round. You are not automatically an Irish citizen by birth if you are born in NI.
The GFA says you can be Irish OR British OR both. It's not clear to people that the choice to be Irish will cost you.