what does the prefix “o” such as
o’connel mean in irish names?
og means young. o’connel is young connel or connel’s child.
Ó originally meant grandson but has more changed to mean descendant of.
With the decline of the Irish language a lot of families dropped the ó.
My family name is Byrne but used to be Ó’Byrne.
Another one…what does kil mean in names (like my own surname)?
IIRC, kil means church
The word for church is cill, which is indeed anglicized as “kil”. However, Kil- in a surname comes from giolla and means “follower of”.
To add a bit more to the O’ discussion: it mostly indicates a member of a particular sept or clan. The O’Byrnes were, as yohimbo says, decendants of someone named Byrne, but they take it as part of their name because he was a prominent man who was the leader of their sept.
So a name like Fiach McHugh O’Byrne would read as: Fiach, son of Hugh, of the sept founded by Byrne. In the course of the forced anglicization in Ireland, most people adopted either their patronymic or their sept as their surname, and dropped the other.
Hey ruadh refresh my memory. what is the -dara in Cill Dara (Kildare). IIRC isn’t it oak or oak grove? Something to do with Brigid also huh?
dara is the genitive form of dair which means oak. So Cill Dara = Church of the Oak. St Brigid founded her monastery there.
I got one for you - Does Fitz, as in Fitzsimmons, Fitzpatrick, etc. really mean decendent out-side of marriage? (i.e. bastard) I heard this somewhere but couldn’t check the source.
The Fitz thing, as far as I know, is supposed to derive from Norman French (like contemporary French “fils = son”) but I think I should leave the cleverer stuff to Ruadh, who seems to be a mine of information at this point. (NOT being sarcastic - being impresssed.)
My parents spoke Irish when I was very small but unfortunately I have retained none of it:(
I’ll add it to the list after German (I’ll get paid to know it)
Thanks, Celyn 
I haven’t paid quite as much attention to Anglo-Norman naming practices as I have to Gaelic, but my understanding is that Fitz- was originally used for both legitimate and illegitimate children, and became an indication of illegitimacy only later on when the use of surnames became more widely adopted - a bastard son wouldn’t be given the proper family name, but would be called Fitz-whoever.
Do not take my word for this, though.