I've been collecting nicknames and name variants from various places as I do my genealogical reserach. Here's what I have so far:
Boys’ Nicknames
Alistair = Alexander
Bartle, Bartly, Bat, Batty = Bartholomew.
Castor = Christopher
Con, Corny = Conor [Corny is from Latin form: Cornelius]
Daniel = David (due to poor penmanship and misreading or miscopying)
David = Daniel (due to poor penmanship and misreading or miscopying)
Darby = Dermot
Edward = Edmond, Eamon (due to phonetic similarity)
Gerald = Garrett, Gerard, Gerailt, Gearoid
Jacob = James (because of Latin form Jacobus)
Kit = Christopher
Lack and Lacky = Laughlin
Neily = Cornelius
Patrick = Bartholomew (through confusion of respective diminutives Pat and Bat)
Roddy, Rory = Roderick
Sandy = Alexander
Owen = Eugene (both being used as translations of the Irish Eoghain)
Peter = Patrick (in Ulster)
Theobold = Tobias (because of shared diminutive = Toby)
Toby = Theobold
Abbreviations
Danl, Dan = Daniel
Jas, Js = James
Jer, Dem = Dermot, Jeremiah [from Demetrius]
Jno, Jn = Jonathan, John
Jos = Joseph
Lau = Lawrence [from Laurentius]
Matt = Matthew
Mich, Michl, Mick = Michael
Nics, Nich = Nicholas
Patt = Patrick
Thos = Thomas
Wm = William
Girls’ Names/Nicknames
Alice = Ellen (due to the diminutive Eily for the Irish names Eilis and Eileen)
Abigail = Deborah (due to the similarity of their respective diminutives)
Abbie and Debbie = Gubbie (the diminutive of the Irish Gobnet or Gobinet)
Ant, Anty, Ally = Anastasia, Anastatia, Anstace
Beesy = Bridget
Bess, Bessie, Betsy = Elizabeth
Biddy, Biddie, Brid, Bride = Bridget
Bridget = Bedelia, Delia, Bessy
Centy = Hyacinth
Eliza, Liza, Lizzie = Elizabeth
Fanny = Frances
Giles = Cecily, Cecilia, Celia, Julia (as renderings of the Irish Sheelagh)
Grizell = Grace (In Ulster)
Gubbie = Gobinet, Deborah, Debbie
Hannah, Hanna = Honora, Johanna, Anna
Honor, Honny, Onny, Noey, Norah = Honora, Honoria
Jane, Jean, Joan = Joan, Jean (all rendered Johanna in Latin)
Jude, Judith, Judy = Julia (due to similarity of their diminutives Judy and Julie)
Jude, Judy, Juggy = Judith
Maggie, Maggyy = Margaret
Molly, Mally, May = Mary
Nancy, Nance, Nanny = Anne, Hannah
Nappy = Nuala, Fionnuala
Nell, Nelly = Ellen, Eleanor, Helen [from Latin Helena, Eleanora]
Noey, Norah, Onny = Honora, Honoria
Peg, Peggy = Margaret
Polly = Mary, Martha
Sally = Sarah, Sorcha
Susan = Johanna (a rendering of the Irish Siobhán)
Latin Forms of Names. Baptisms and marriages were recorded in either Latin or English, never in Irish. Generally, where English was more common English was used and Latin was used in Irish speaking parishes. There is however, no consistency. The Latin version of the first name was given while the surname and place name were still written in English.
For Boys:
Carolus = Charles
Cornelius = Latin form used for the Irish name Conor (Conchobar), which has also been anglicized as Cornelius, Conor, and sometimes Neil or Neily.
Demetrius = Latinized form of the Irish name Diarmaid, which has been anglicized as Dermot, Jeremiah, Jerome, Jerry, and even Darby.
Dionysius = Latinized form of the Irish name Donncha, which has been anglicized as Denis
Donatius = Daniel
Eneas = Latinized form of the Irish name Aonghus, which has been anglicized as Angus
Eugenius = Latinized form of the Irish name Eoghan, which has been anglicized as Owen
Gulielmus = William, Liam
Hugones = Latinized form of the Irish name Aodh, which has been anglicized as Hugh
Nigellus = Neil, Niall
Ioannes = Latinized form of the Irish names Seán and Eoghan, which have been anglicized as John and Owen
Jacobus = Latinized form of the Irish names Seamus, which has been anglicized as Jacob or James
Johanes, Joannes = Latinized form of the Irish names Seán and Eoghan, which have been anglicized as John and Owen
Kyrianus = Kieran
Patricius / Patritius = Patrick
Randolphus = Randall, Randolph: Ralph
Timotheus, Thaddeus = Tadgh, Thady, Timothy
For Girls:
Anna = Ann(e)
Brigida / Brigita = Bridget
Honoria = Hannah, Nora, Norry
Ioanna = Latinized form of the Irish name Siobhán, which has been anglicized as Johanna, Hannah, Joan, and Jane
Johana = Latinized form of the Irish name Siobhán, which has been anglicized as Johanna, Hannah, Joan, and Jane
Juliana = Julia
Margarita = Margaret, Peg (Peig is actually the Irish name for Margaret)
Maria = Latinized form of the Irish name Máire, which has been anglicized as Mary or Marie