Northern Portugal and Galicia are today two different entities, though in the Roman period they both constituted the Roman province of Gallaecia.
As far as having any current Celtic-speaking populations, I can assure you that nobody in Portugal and Spain speaks any Celtic language; all languages spoken here come from Latin.
Regarding the Celtic descent, well, that's more problematic. When the Romans arrived to Hispania, a Northern Portugal and Spain were occupied by Celtic-speaking populations and some Celtic words still survive today. However, this does not prove that there was a large (if any) influx of Celtic migrants into this area. After all, we are all speaking English in this forum, but that is not the native language of many of us. In my opinion, I think it is more likely that Celt was a lingua franca in large parts of Europe in those days, due to a little know process of cultural transmission. It was the Celtic culture that was moving around Europe and not a people.
Finally, the main reason why more recently there has been an emphasis in Northern Portugal and in Galicia about their Celtic heritage is due to political reasons, that is, it is about creating a common past between both areas so that, together, they can resist better the centralisation attempts of the capital cities of both countries.