Sorry, I have not read the whole thread but I can tell you categorically that those men mentioned in the song which begins "Ten years before I saw the light of morning" was the Fifteenth International Brigade https://en.wikipedia.org/wiki/XV_International_Brigade which was mainly composed of English-speaking battalions and columns.   Those are only some of the known Irish who fell fighting fascism in the territory of the Spanish state.

In the English lyrics of Christy's song, he says "Fifteenth" and its what is printed in his songbook. My guess is that it's what he sings too although on a recording it is possible to mishear it as "Fifty".

At a guess, Christy took advice from someone who misinformed him about the correct Spanish for Fifteenth, which would be La Decimoquinto and told him "Quinta" which means "Fifth".

Decimoquinto has five syllables instead of the two of Quinta. However, one can get over that by singing Quinze (fifteen) which is not strictly grammatically correct and which has the misfortune of being the title of an older Spanish Civil War song.

The reference to the air of Red River Valley does not belong in my view to EITHER of those songs but to another, The Valley of Jarama, composed by a fighter of the British Battalion, Alex McDade which does go to the air of the Red River Valley. This song was adapted to cover the Lincoln Battalion by Woody Guthrie and also changed somewhat for the British Battalion in later years.

I don't know where Christy got the air for HIS Viva La Quinze Brigada which in my view is one of the best songs dealing with the period and a great song in its own right.

Viven!