is this also used for "more or less", or would one use another phrase/ word-order?
The simple answer is that it wouldn't be translated into Irish - you'd probably have to find a different way to express the same concept. níos mó can mean "bigger" or "more" - Tá an madra níos mó ná an cat means "the dog is bigger than the cat".
Similiarly, níos lú means "smaller" as well as "less" and "fewer"
Tá an carr seo níos lú - "this car is smaller"
Tá níos lú carranna anseo - "there are fewer cars here"
Tá níos lú agam - "I have less"
Of course now I'm dying to know how one would express the highly idiomatic phrase, "Less is more!"