This is a very, very common thing amongst Irish people when describing a person, place or thing. Because sometimes we can't articulate how large or small an object is, or the skin tone of an individual or even the heat of a local curry dish - we use this a lot! We basically just say the word twice and the person who gets the info knows exactly what we mean by tone alone.
Used to describe anything good!! can be used for something bad but generally good!! similar words are class.. deadly it is also used to agree with a question or just answer with stout!!