"There will be two cats there."
Translation:그곳에서는 두 마리의 고양이가 있을 것입니다.
Both are correct. The order of the words helps to determine the focus and emphasis of the sentence. The later in the sentence a word is, the more important it is.
So, 두 마리의 고양이 puts more focus on the cats, rather than, let's say, dogs.
고양이 두 마리 puts more focus on the number of cats.
The "의" following the 마리 just connects the counter with the corresponding noun.