I agree completely with what Kevin says. I live in Co. Antrim and have a friend named Ellen on her birth certificate but she’s always been Helen to her friends. (Something we only discovered when a flight ticket bought by someone else as a surprise treat didn’t match the name in her passport). Names also change when people move to live somewhere else. Ellens who go to Scotland usually become Helen there (presumably because it’s more common. I don’t really know). Other examples of interchangeable forenames are: Jean & Jane, Jenny & Janet, Nancy, Agnes & Ann are all interchangeable, Sally & Sarah, Sheila & Julia, Mary & May, Peggy & Margaret, Peter & Patrick, Sean & John, John & Jack, Edward & Edmund, Henry & Harry, Robin & Robert, Roy & Robert, Kathleen & Catherine. (The famous Scottish poet Robert Burns was never known as Robert to his friends. He was always Robin to them).
Then I could explain about Irish names such as Eamon and Edward, Eoghan & Owen. But life’s too short. Google it.