To have British nationaliy status, one also has to be a British subject, and most citizens of Ireland other than those born in Ulster, are not British subjects, even if they are also eligible, and have, British citizenship,... although some Irish citizens, who are British subjects, may not also have British citizenship.
Some holders of British passports, are listed in their passport as a British subject, but that kind of passport has additional visa requirements in some countries, which are not required if the person has a British passport which lists them as being a British citizen.
It's a very complex legal system with very specific and detailed legal rules and requirements, and at times it has even caused confusion and misunderstanding within the British government,... and that's even before the E.U. and breaksit situations are also taken into account,... as the woman above from Derry/Londonderry/Doare and her American born husband have found out.
