Isn't Castelhano just Portuguese for Castillian? (Castillian is the same language as Spanish, though some people use it nowadays for "Spaniard Spanish")

Actually, I had the same problem.
While I was listening to it (without lyrics), I couldn't pinpoint if it was supposed to be Spanish or Galician (it looks like Spanish with some Galician words smattered, a kind of mix tha'ts very frequent in some zones of Galicia). When I read the lyrics were from Zeca Afonso, I assumed it was Portuguese (I had to look up the meaning of "percar"), but, as both of you pointed out, this is very far from Portuguese. Galician-Portuguese is just another bad guess for the same reason.

I didn't really know about the existence of mirandês (now I read that name again, I think that it popped up in a topic about plurilingual countries, where sacdegemecs and I participated). Looking at its article in Wikipedia, this looks too close to Spanish to be a good sample of mirandês, but, of course, you'd want to speak to a real speaker.

So, if this song existed prior to Zeca Afonso, most likely it's a somewhat archaic Spanish or a mix of languages, transcribed to Portuguese ortography in any case. Don't know what to set the language tag to, then; either Spanish or "other"?