As Jeniadams says in the very first answer, ''It just depends on the individual how they want their own name to be said''.
Barry - The monks who first wrote Gaelic had a problem. Their alphabet contained only 18 letters, yet the language held some 55 different sounds. Rather than invent new letters they added in existing ones to modify the pronunciation of adjacent letters. They had no precedent, yet made an excellent job of phonetics if their rules are followed. Unfortunately it left Gaelic with some of the most complex spelling of any language! Have a look
here.
Swedeheart - Take the English name Catherine. It has a short 'i'. Even today, the word is often pronounced as 'Kath-rinn', so drop the first 'e', In Gaelic, 'th' is pronounced as the 'h' in 'happy', so the 'h' in Cathrine is dropped to become Catrine, This could be 'Kat-reen', so the monks stuck in a modifier 'o' to show it's 'Kat-rinn'. Next, think of the European name Katrina, or the French penchant for pronouncing the final 'e' on a word. Perhaps that's how it used to be voiced in Britain, and where the final 'a' comes from.
Maybe QM could provide an expert comment on this?
Anyway, The first answer's the best!