This anomaly in British Nationality law relates to those born in Ireland before 01.01.1949. This is a form of British Nationality that was designed to become redundant in 1949.
It allows such people to register now as British Subjects. In addition, where the person has a parent born in Ireland before 31.03.1922, or any Grandparent born in Ireland, then the person can register as a British Subject and claim the Right of Abode.
In circumstances where the person has no nationality, this British Subject status can be upgraded to full British Nationality.