One of the more interesting Irish naming practices was equivalent naming. Like a lot of Irish practices, this was a way to get over stupid or well-meaning interferences into normal life.
When the Reformation and Counter-Reformation came along, there was a lot of embarrassment in Europe over all the weird local saints’ names, and the lack of historical documentation for even some famous saints. (Not that this meant they weren’t historical; and afterwards researchers often found tons of info, of course.) So a lot of Protestants decided to stick to Biblical names only, while a lot of bishops felt that Catholics should stick with saints of the universal Roman calendar only.
This was a problem in many countries who had hundreds of saints commemorated on (perfectly legal, bishop-okayed) local calendars, but only a few or none on the universal calendar. Ireland didn’t have a single saint on the universal calendar at the time, although saints like Brigid and Patrick were known and loved all across Europe during the Middle Ages. It just had never been needed, since Irish priests, Irish-missionized cities, and Irish-founded orders had used the relevant bits of the Irish saint calendars for their local diocesan saint calendars.
So when young, fervent, rules-obeying priests came back to Ireland, you had the strange situation of Catholics being told that if they wanted their kids to be baptized, they couldn’t name their children after the same Catholic saints that had been their family patrons for over a thousand years. It had to be a saint of the Roman calendar instead.
At the same time and even earlier, you have a lot of the English legal system insisting that Irish people have names that non-Irish people could spell and pronounce in an English way. The Roman calendar saints often had weird names, but the English could spell them and make them out.
So instead of Conn or Conor, there are suddenly hundreds and hundreds of Irishmen named “Cornelius.” Everybody at home still called them Conn or Conor or Connie; but technically, their name was Cornelius. Similarly, there are suddenly a lot of men named “Timothy” who are really “Tadhg” (pronounced Ty or a bunch of other ways, depending on where the Gaelic’s from). This is also why Moire (the Gaelic form of Mary used for names, as opposed to Muire that was only for the Virgin Mary) often gets written down as Maura or Maureen — it’s technically being named after St. Maura the martyr, although everybody knows perfectly well it was always used for Mary.
So today, I found out that a large number of Irishmen named “Jeremiah” were actually named “Diarmuid.” Nothing against St. Jeremiah or his prophecies, but this makes a lot of sense. Diarmuid (also spelled “Dermot”) was a very popular name for a long time, and these things don’t just disappear without a trace.