Hi Katja,
How wonderful that you'll be visiting the UK and Ireland! It's great that you remembered to get your ETA for your visit to the UK.
Everyone who does not need a visa to enter the UK and wants to travel to the UK will need an
ETA assigned to a valid passport. Dublin and Galway are in the Republic of Ireland, and you do not need an ETA to visit these. Depending on where you are from, you might need a visa, though. You can read more about
visas and passports here.
It's not entirely clear to me from your message if you are from Canada or somewhere else? If you let me know, I'd be happy to give you more detailed information.
If you are from a country that requires you to have
a visa to enter the UK, you won't need an ETA to visit the UK.
I hope this helps!
Noortje