This web can be useful to find mass times although it isn't always updated:
http://www.misas.org
Type the name of the town/city where it says "Buscar misas en la localidad" (on the top of the page) but be sure the church you are looking is in the town you are staying and not in a more or less nearby town in its municipality.
Some of the results are:
Tui: Parroquia de San Francisco
http://www.misas.org/p/parroquia-de-san-francisco-tui
O Porriño: Parroquia de Santa María de la Concepción
http://www.misas.org/p/parroquia-de-santa-maria-de-la-concepcion-o-porrino
Redondela: Parroquia de Santiago
http://www.misas.org/p/parroquia-de-santiago-redondela
Caldas de Reis: Parroquia de Santo Tomás Becket
http://www.misas.org/p/parroquia-de-santo-tomas-becket-caldas-de-reis
Padrón: Parroquia de Santiago Apostol
http://www.misas.org/p/parroquia-de-santiago-apostol-padron
I don't quote results for Pontevedra and Santiago de Compostela because there are too many parishes on both cities and I don't know which ones could be more convenient for you.
On the towns that I quoted, there are other churches where you could attend mass so ask locally for options.
Don't forget to verify locally the mass times on your specific dates because the linked web isn't necessarily fully updated (and because there can be differences among parishes on when the summer times apply).
Invierno=winter
Verano=summer
Laborables=work days
Vísperas=the day (or the afternoon/evening) before Sundays and before Holy Days of Obligation. On a víspera mass, they make the 'same' mass they'll make next day. Some víspera days, on some parishes, they can have, for example, the mass of the day in the morning and a víspera mass on the evening.
Festivos=Sundays and Holy Days of Obligation (sometimes it might apply on Public Holidays that aren't Holy Days of Obligation).
