Were the names Rebecca and Elizabeth ever used interchangeably, specifically in Ireland? I am doing some family research, and it seems that these two names may have been used to refer to the same person.
It is not at all unusual, particularly in the nineteenth-century to find people who were known by names other than the ones they had been given in baptismal records, and, after compulsory registration began (in Britain 1853) on birth certificates. They may have been known by their alternate name only to certain people, or at certain times of their life.
In my own genealogy I have two or three male persons who were always known by the name of their father, which was different to their own.
But I can't think it can possibly be the case that Rebecca and Elizabeth were generally interchangeable. In the Bible those names were held by quite separate people.
Having done considerable amounts of genealogical research in my time, I am not aware of any names (other than recognisable short forms) which were generally interchangeable with others.
The names are not alternatives of each other; but it's quite possible for both to be used of the same person. The playwright Shaw was "George" (or "Sonny") to his family but "Bernard" to the literary and theatrical world and on his title pages; only ignorant journalists called him "George Bernard Shaw". My own wife is known by her first name to her family, to government agencies, to billing and Human Resources offices, and to direct-mail and telephone advertisers; but in high school she elected to adopt her middle name among her friends, and she has used that in all professional and social contexts ever since.