That 'five' 'fifteen' mystery still did bother me. So I did a little research project:

Christy Moore has in both of his songbooks 'fifteenth' in the lyrics, on his website he has 'fifth' (the lyrics printout on the Ride On LP says 'fifty' but we don't take that serious). The title on the recording is given as 'Vive la quinte brigada' but that has been corrected menawhile in print. I've listened last night and I'm sure he sings 'fifth'. So who's right, Wikipedia or Christy Moore. My newest guess is that they are both right for it is just a matter of counting.

From the Wikipedia article about the International brigades:

The first International Brigade, the XIth Brigade (numbered XI, next to the ten mixed brigades of the Spanish regular army)...

So the international brigades started with the number 11, therefore the 15th brigade altogether was the 5th of the international brigades. And this brigade (in which Frank Ryan's men fought) was the '5th international brigade' if one counted only the international brigades or the 15th international brigade if one counted all brigades and used the 'international' only as an adjective to 15th brigade.

Wolfgang