I have been trying (for so long now that I've even forgotten why exactly..!) to find out the etymology of the word "ceacht" (lesson) -- ach ní raibh an t-ádh orm. Can anyone help?

I think it stems from the Latin 'praeceptus' - 'proiceapt' > 'proiceacht' > 'ceacht'.

A Shéamuis, you're amazing. You wouldn't believe how long I've been trying to get someone to give me a serious answer to this question -- and yours convinces me!

All I have to do now is remember why I wanted to know this... Go raibh míle míle maith agat!

You are very much on the right track. But "ceacht" actually comes directly from Old Irish "aiccecht" (sometimes written "aiccept") from Latin "acceptum".

Did the Irish derivitaves of 'acceptum' and 'praeceptus' become confused?

I don't think so. Acceptum = recipe; close enough to a lesson, I guess.

Dála an scéal, is dócha gur chuala tú tagairt don leabhar Auraicept na nÉces? http://en.wikipedia.org/wiki/Auraicept

Séard atá san fhocal "auraicept" ("airaiccecht" in DIL) ná an réimír air- móide aiccecht. "Primer" nó "first instruction" an chiall atá leis.

A mountain of thanks to you too, Dennis. The scholars are out in force today!