The judges found that the relevant act was the British Nationalities law and that had not been replaced by the Good Friday Agreement in legal terms. The Good Friday Agreement allowed for residents of Northern Ireland to choose Irish or British citizenship but the British government never gave it the force of law.
"This decision from the court today is a disgracefully retrograde step," Mary Lou McDonald, Sinn Fein president said.
Foreign Minister Simon Coveney also voiced his anger at the finding that people born in Northern Ireland are automatically British.
Irish deputy leader Simon Coveney.
"Citizenship and Identity provisions are critical to the Good Friday Agreement (GFA)," he said.
"The UK Govt has pledged to review rules around citizenship and deliver a long-term solution consistent with GFA. An outcome is urgently needed and I will raise this again with Secretary of State for Northern Ireland tomorrow."
The British Home Office -rejected the application stating it considered her British because she was born in Northern Ireland. They told her to reapply under British citizenship or take up Irish citizenship and renounce her British citizenship.
She made clear that she never considered herself British, so how she asked could she renounce citizenship she never had?
She won her first appeal but the Home Office appealed to a higher court based in Belfast and won.
Sinn Fein President, Mary Lou McDonald.
McDonald TD said "The Good Friday Agreement is crystal clear on this in terms of citizenship. Emma De Souza is an Irish citizen and it is disgraceful that she should have to go to court to prove it.
"The British government is a co-guarantor of the Good Friday Agreement, but yet it is trampling all over it.
DeSouza vowed to fight on stating everyone in Northern Ireland had the right to their own identity.