Dia daoibh!
I am a bit confused regarding surnames. I have read, for example, " Padraig Ui Neill". Is the English version Patrick O'Neill? I mean is "Ui" in front of a surname the same as O'?
Also "mac" is Irish for son, so what does O' stand for? The same goes for "Ni"in front of a woman's surname.
Thank you in advance for anybody taking the time to answer my posting.
Slán agus Beannacht
Isabelle

Dia's Muire dhuit a Isabelle,

To the best of my knowledge 'Uí' is the original form, and 'O' or 'Ó' is the version 'as Béarla' (in English).

'Ní' and 'Nic' are used as prefixes for womens names, depending on whether they are 'maiden' or 'married'; but as for the correct order, I'd better let others assist you. I'll look it up and post some more soon.

Ádh mór ort,
Fintan

from what i have read o' means grandson of'.

'Mac' means son; Ó means grandson or descendant. 'Mac agus ó' son and grandson. 'Níl mac ná ó aige' He has no one to succeed to his name. 'An dá ó' second cousins.

Seán Ó Murchú = Sean Murphy
carr Uí Mhurchú = Murphy's car
Síle (Bean) Uí Mhurchú = (Mrs.)Sheila Murphy
Síle Ní Mhurchú = Miss or Née Murphy

Tarlach Mac Cába = Terence/Tarlough McCabe
teach Mhic Chába = McCabe's house
Treasa (Bean) Mhic Chába = (Mrs.) Therese McCabe
Treasa Nic Chába = Miss or Née Therese McCabe
(Nic because of the 'c' in Cába -- otherwise it would be Ní)

The form that is equivalent to 'Mrs.' is really the genitive form that indicates belonging to. Traditionally, Irish women often continue to be known by their 'Ní/Nic' names after marriage.

As for Uí it is sometimes used where other places Ó is used, especially with ancient names, and Uí Neill is as old as the get.

What Padráig Uí Neill means is Padraig of Nialls family/tribe, the Niall in question being Niall of the nine hostages.

I'm not sure if there are any hard and fast rules as to when Uí/Ua is used and when Ó. I suspect it is a matter of custom rather than grammar.

There are lots of forms of this word. Here's more info., mostly from the dictionaries.

'Ua' is a variant of Ó which people sometimes choose over Ó for their surnames, presumably to give it a more ancient look: Ó Cuana or Ua Cuana (Ua Cuanaigh, I think, is what a friend by that name used).

'Uí' is interesting because it is not only the genitive singular of 'ó'; it is also a nominative plural used in historical names and terms: grandsons, descendants, issue. That's how Uí Néill as a stand-alone unit comes about: Uí Néill = the O'Neill's, the descendants of Ó Néil (of the nine hostages).

The preferred nominative plural in the modern dictionaries is, however, 'óí' (good to remember for Irish-language Scrabble (t) games). A óí agus a iaróí = his grandchildren and his greatgrandchildren. Other nominative plurals are uaí, óthanna and óigheanna. Especially in some phrases, these forms mean cousins or friends: Ní siad ar na h-óigheanna (or ar na huaibh) le chéile. They are not on good terms; they are not related.

There is a dative plural, uíbh (with variants) that you see mostly in placenames: Uíbh Eachach = Iveagh

Thank you all. You have been very helpful.
Isabelle.