Irish nationality law favours British people born in Northern Ireland. According to Irish law, if a person is born on the island of Ireland on or after 1 January 2005, that person is entitled to be an Irish citizen if at least one of his or her parents is British1.
Under the Good Friday Agreement in 1998, it was “the entitlement and birthright of every person born in the island of Ireland, which includes its islands and seas, to be part of the Irish nation.” Thus, anyone at all born on the island of Ireland was entitled to be Irish. The Irish Nationality and Citizenship Act 2004 changed this with effect from 1 January 2005, but British people born there are not effected by the change.
We understand that the intention of the Good Friday Agreement was to allow a person born in Northern Ireland to be able to identify as Irish or British or both, such that everyone is happy with the available possibilities. When that Agreement was negotiated, the consequences of Brexit were, of course, not contemplated.
Being entitled to Irish citizenship is not quite the same as having it. The baby becomes an Irish citizen if “he or she does any act that only Irish citizens are entitled to do” or such an act is done on their behalf. This might simply be applying for a passport. In contrast, British citizenship is automatic.
While a child born in the Republic of Ireland to at least one British parent is also entitled to Irish citizenship, there are excellent NHS hospitals in Northern Ireland that we Brits may access. Post-Brexit we expect to have to pay for non-emergency healthcare in hospitals in the Republic of Ireland. Thus, Northern Ireland is clearly a preferable destination for British people wanting their baby to have EU citizenship.