Why does it say "an madadh" in the "choose the right word combination" exercise? What's the difference with an madra?
Madra and madadh are two different words for “dog”.
I know your comment was two years ago but different parts of Ireland have different variations of Irish. (especially Ghaeltacht areas) For example, my Irish teacher spells "madra" as "mada" and she is from a ghaeltacht area in Galway.
Is it "cú" in use in Irish gaelic today?
Cú is the Irish for "hound" - it isn't a generic replacement for "dog".