There are two brigades documented by their songs, the quinta = 5th and the quince=15th.
Q just gave the link to the 5th brigade song.
Here is the link to the 15th brigade song
Both songs have some similar stanzas; nothing new in the history of folk songstransformed for special units or events.
Both songs are sung to the tune of the Ebro Army (the Carmela).
To answer future possible questions: There also is a 5th regiment, sung to the tune of the old Spanish song, or dance, "el Vito" (St Vitus' dance)

More informations about units of the Spanish Republican Army may be found here: Foro por la memoria". Use the search box, upper right.