"A skyscraper of Korea"
It's pronounced /마철루/. ㄴ+ㄹ (or ㄹ+ㄴ) in the 받침 turns into double ㄹ.
As for the double ㄴ the 받침 combinations are: ㄷ + ㄴ ㅌ + ㄴ ㅈ + ㄴ ㅊ + ㄴ ㅅ + ㄴ ㅆ + ㄴ All of the above become double ㄴ when pronounced.
However there are times when ㄹ turns into ㄴ, so that's what you might have heard about? For example combinations where we have ㅇ+ㄹ in the 받침 the ㄹ is pronounced as ㄴ. For example the word 장래 (meaning: future) is pronounced /장내/