Buenas tardes Shui e Inma ...
It might be worth considering ... >> ?
1. > "As many Irish people emigrated to the United States..." [because that corresponds better with the 'emigraron' in your Spanish translation].
2. [Debatable !] > I first wondered whether "Halloween is really an ancient Irish holiday" might have been more helpful to us, rather than saying "... ancestral..."]... In Castillian, 'ancestral' is indeed sometimes used as a synonym for 'antiguo' - but perhaps there is a very slight difference in Englsh? Eventually, however, I could see that the use of the word 'ancestral' in that context was at least pointing us in the right direction.