[samibe]

I would probably set the nationality under Collections (Scottish, Irish, etc.). Then I'd set the type under Genre (Songs, Airs, Waltzes, Jigs, Reels, etc.). 

If you batch import a folder of Scottish Waltzes, then you can set the collection to Scottish and the genre to Waltzes.
If you have a song or tune that is multinational or could fit in multiple genres, you can add additional tags to the collections field or the genres field in the song editor.

Also once you have your library loaded, there are multiple avenues to find a particular group of songs/tunes. For Scottish Waltzes, go to collections tab, select Scottish, filter genre by Waltzes (or start typing Waltzes into the search bar) and the resulting list of songs/tunes will fit both criteria. 
Or you could go to the genres tab, select Waltzes, select Scottish under the collections filter (or start typing Scottish in the search bar) and you'll probably get the same list. 
Or from the songs tab, you could select Scottish under the collections filter and Waltzes under the genre filter or select one of the filters and type the other in the search bar. 
If you find that you regularly use the same combos (Scottish Waltzes) and would like to save time, you could filter the list using one of the methods above then select all of the songs in the filtered list, click the three dots in the upper right corner, select "Create Collection from Songs", and name the new collection 'Scottish - Waltzes'. This will allow you to go to the collections tab and select the 'Scottish - Waltzes' and get the fully filtered list. 
You could also do this during batch import too (in the collections field, type 'Scottish', hit enter, type 'Scottish - Waltzes', hit enter, then in the genres field type 'Waltzes')