I'm a massive fan of Peadar Ua Laoghaire and have read most of his works, so I'll just say a bit, I hope people find it useful.

Peadar Ua Laoghaire's books go roughly like this in difficulty, in my opinion, easiest first:
Sliabh na mban bhfionn
Mo Scéal Féin
Eisirt
An Craos-Deamhan
Don Cíochóté
Niamh
Cleasaidhe
Séadna
Táin Bó Cuailnge*

Peadar Ua Laoghaire wrote in a very pure form of Cork Irish, with lots of grammatical forms and subtleties that aren't used at all today. There's a lot of phrases and words in his books that had a separate meaning for him, but mean the same thing today. As well as a lot of words that have fallen out of use.
However Séadna and Cleasaidhe take this a step further and he in the first he is attempting to sound like his uncle born in 1780, so the Irish is even more archaic. In Cleasaidhe, I think he's trying to sound like an old Bardic story so he has things from the 16th century.

In general the stories are really fun and have a real medieval flavour to them. My favourite is Séadna, but Niamh probably has the best story. Personally I'd say Mo Scéal Féin or Niamh for an online group, or Séadna if we're feeling brave!

(* Táin is more modern than Séadna, but there is a lot of subtle uses of phrases and idioms.)

You might find something useful here.
http://www.scoilgaeilge.org/lessons/seadna/seadna.htm

I personally would like to read the original unabridged, unstandardized version (with modern spelling, of course). They are readily available from Litríocht and so are the CD's.