I think people get abit too hung up on the P/Q celtic divide. Looking at Welsh now obviously it seems quite alien compared to Irish but when you understand the orthographical differences you can see words that have common meanings. They reckon that the spilt between Irish and Welsh could be about 2,500 years, given that neither was written for a further 1,000 years it's not surprising that they look different, but in sense the basic P/Q divide is like the "High German" shift. For example Dutch only shifted th ->d but it didn't do for example d -> t (English: day, Dutch: Dag, German: Tag) or t -> ss (english that, German dass
As for Cruithne I can't really comment as I don't know enough though the medieval Dal nAirde are purported to be their successors. If they had originally been p-celtic then they had undergone language shift. Given the closeness of the two languages at the time this wouldn't have been too hard (lot easier then shifting Irish to English for example).
The Connachta if you believe the ancient Genealogists are descended from Conn Cétchathach (Conn of the 100 battles). Who supposedly lived in the 2nd century around the time of Marcus Aurelis and who ruled the northern half of Ireland (line from Galway to Dublin) as Leath Cuinn (Conn's half). The southern half (leath Mugh) been ruled by the ancestor of the Éoganacht: Mug Nuadat
However alot of Irish history at this stage is purely conjucture if not outright mythology in it's own way. It would seem that the history books were rewritten by the Uí Neill for dynastic propaganda.