This (with the parallel discussion on 5th/15th Brigades) is a fascinating thread: I wish I'd come to the party earlier.

I am dubious whether what follows is of any great help; and I realise that I am talking to experts; but may I make a couple of tangential points?

First, I guess that Christy Moore song owes something to Sean Cronin's 1980 book on Frank Ryan (now only available second-hand, and frequently at a price beyond rubies).

Second, the confusion between 5th/15th may be quite straightforward. The Fifth Regiment was the Communist Party's militia. Chapter 27 of Hugh Thomas's history of "The Spanish Civil War" considers why this was so effective a fighting force:
"... the most formidable of the Republican forces in the Sierras was that founded by the Communist Party, the Fifth Regiment. It was so named because there had customarily been four regular regiments stationed in Madrid, whose military organization, rather than the revolutionary and enthusiastic indiscipline of the militias, the Communist party from the start tried to emulate.
"The force was based upon the Communist-Socialist Youth, but others joined as a result of the recruiting drive led by La Pasionaria ... By the end of July [1936], nearly 8,000 members of this group had gone to the front. By early September, 30,000 persons had been members of it at one time or another, many having passed on to other groups. In addition to its regular organization, the Fifth Regiment adopted the use of political Commissars employed by the Red Army in the Russian Civil War, with the declared aim of making clear to the soldiers what they were fighting for."

It occurs to me that "Viva la Quince Brigada" is in Spanish. Were it from the 15th Brigade, it would more likely be (as the other songs are) in one of the languages of the Brigaders

Compare the size of the 5th Regiment with what, in Appendix Three, Thomas says on the size of the International Brigade:
"The total number of foreigners who fought in the International Brigade was about 40,000, though the Brigades never exceeded 18,000 at any one time.... The largest national group of volunteers were the 10,000 French, of whom 3,000 were killed. Germany and Austria together contributed about 5,000, of whom 2,000 died. Italy came next with 3,350. The United States contributed about 2,800. Of these about 900 were killed. There were about 2,000 British volunteers, of whom over 500 were killed and 1,200 wounded. There were about 1,000 Canadian volunteers, 1,500 Yugoslavs, 1,000 Hungarians, 1,500 Czechs, and 1,000 Scandinavians. The other 3,000 volunteers came from what was claimed to be fifty-three nations."

One further complication: the Irish involvement (which takes us back to Christy). The original "unit" of just 80 went out to Spain in December 1936, expecting to support the Basques, whose cause was espoused by the Republican Congress. The intended commander was to be George Gilmore, but his broken leg (caused by a crash landing visiting Bilbao) promoted Frank Ryan. There already were Irish fighting in Spain, with the Thaelmann Battalion (Tomas Patton from Achill and Wiliam Barry from Dublin and Mebourne died in the defence of Madrid in December 1936).

Anyone trying to work out the list of names in Moore's song could usefully refer to Mick O'Riordan's paper at CLICK HERE