Do not decide on what souvenirs to buy before you are in the country and have seen what is available.
In each of the countries you list music is a good choice, and you might want to download some music for each before you are traveling.
Even if all have their own kind of traditional music, Ireland is specially known for that.
If you like clasical music, Germany and Austria have more than a fair share there, but again all have some famous componists.
Or go for the kind of music you would always listen too and search on base of country this time.
When you are traveling you will be swamped by the souvenir shops, you can easily see what is the most popular gift in each country, but select your gifts based on what the person it is for would like best.
(If your grandmother is from Germany, a mug with 'best grandmother in the world' in german is much better than a beer tankard in miniature.)
And better not buy big souvenirs till your last country of the trip. They get heavy, and for the people getting them it makes no difference from which country they get them, as long as they know you have thought of them when having fun.
For yourself it is nice to have a souvenir you can actually use, I like T-shirts of the places I have been, and they are available in every country.
And if I need something, I rather buy a special thing that is just what I need, like when I was in Australia I needed a little purse just for my coins, I still use it every day. Much better than a koala stuffed animal that will sit in my house getting dusty.