Far be it for me to really know (on the wrong side of the pond), but I did read this in Ciaran Carson's most excellent book, Last Night's Fun: In and Out of Time with Irish Music:

"In my teens in Belfast, a ceili was a social event imprimatured by the Catholic Church where boys and girls met each other under close sacerdotal supervision and pracitsed minimal-contact dancing; the best part was where you got to swing the girl. The only drinks were soft, though some boys were known to smuggle in flat half-bottles of "British Wine." It was a far cry from the ceili I encountered later on in life."

He goes on to say:

"In its primary sense, ceili does not necessarily include music. Father Dinneeen's Irish Dictionary has `an evening visit, a friendly call', with `an evening of musical entertainment' secondary, and that probably in deference to those evenings formally devised by the early Gaelic League. The word derives from ceile `a companion'." (Sorry, I cannot remember the html to make the diacritical mark over the first "e")

Then he explains that in more depth and that Dinneen's entry is a full column long.

He also makes note that "in rural ares, ceili denotes a visit or series of visits; a way of passing the time of day or night. Anglicised as kayley, as is current in West Ulster, especially, sometimes as a verb -- recalling an absent friend, one might say, `I used to kayley with him.'"

One term he uses throughout the book but doesn't really explain except with one small remark is fleadh and one close to it,fleadhana(sp?). It seems it means a musical competition, but I'd love to hear more about it, too.

Great start for a thread. Thanks!

kat