Hi, I just got Gnás na Gaedhilge (1940) by this professor of Irish. It is written in the gaelchló, with the small capital Rs and Ss rather than the long r's and long s's. It seems quite good. But I can't find any background on this man, other than to say he was a very reputable professor of modern Irish at UCD. Where did he come from? Was he a Munsterman?

He was a Munsterman indeed: born in Kinsale in 1884. Your best source of information, to start with anyway, is 'Beathaisnéis a Dó 1882–1982' (page 76, Breathnach agus Ní Mhurchú,, An Clóchomhar). There's a brief account on his life there and mention of different other sources and publications, both written by him and written about him.
I've come across Ó Cadhlaigh in research in the past. There is a major editorial error in 'Index do Irisleabhar na Gaedhilge 1882–1909' (Ní Chiaragáin, 1935) for example. There's a long list of poems attributed to him in this book, most of which, in actual fact, are the work of Donegal poet Pádraic Ó Beirn.
Note that this isn't a mistake on Ó Cadhlaigh's part (who's seems to have been a widely respected scholar) but rather an error made by the editor(s) of the Index.
Best of luck with the research.

Thanks Umpáin. I found mention of him in Phili O'Leary's book GAelic Prose in the Free State. He was born in Kinsale, and become Professor of Irish at UCD and is noted for Gnás na Gaedhilge as well as his book in Irish on the ancient Irish King Cormac mac Áirt. I believe his mother was a Munster native speaker, but he himself learned Irish from O'Growney's books. I am wondering if he is related to the Ó Cadhla family of the Waterford Gaeltacht, as there is a letter to him from Pádraig Ó Cadhla (who translated Alic in Wonderland into Irish in 1922) in the National Library of Ireland manuscript archives.