In relation to John/Jack, etc, government records like birth marriage and death certs tended to use the formal name - so someone who was known to all his friends as Jack or Mick or Sandy would be listed as John, Michael or Alexander; Cecilia was sometimes spelled Cecelia, and Frances became Fanny, Mary and Margeret became May and so on.
Then there's the question of names in Irish - as people became more intense about the need to separate from the British Empire, plenty of people used the Irish version of their names, so John might became Sean, or the older spelling Seaghan, and James Seamuis and Peter Peadar, and often Cecilia would become Sheila or Sighle, Florence would be Bláth or Bláithín… And the later death records, after independence from 1922 on and especially from 1949 on when Ireland became a republic, often use the name that was customarily used - so someone born Peter Kearney in 1883 was registered at death as Peadar O Cearnaigh in 1943.
