Thank you Reg Hall! His book "A Few Good Tunes" -- maybe I have got the title wrong, something like that -- has discographies for a number of traditional Irish musicians. Hall documents and describes Ní Scolaí as one of the few performing artists who would program a concert of largely Gaelic lyrics in a formal concert setting right in London around the time of the Second World War. I ought to go back to the online book and look up Mr. Hall's exact words on this.

Here, from Mr. Hall's book, is information for which I have been digging for weeks.

On 20 August 1938, singer Máire Ní Scolaí and pianist Duncan Morrison completed a session in the London recording studios of His Master's Voice. Mr. Hall can positively state that the session included these titles:

My Bonny Irish Boy
Caoineadh na dTrí Muire
Cuaichín Ghleann Neifín
'S é Fáth mo Bhuartha
Eibhlín a Rúin
and something that looks like "Aghadoe"

At that moment, Ní Scolaí would have been just 29 years of age, so physically and vocally she must indeed have been in her prime.

A shorter HMV recording session in London was squeezed in, the following year, sometime around March (1939). The titles recorded were:

Dun Do Shuile
'S Óró Mhíle Grá
Lúibín Ó Lú