Do we know anything about this planet and those isles / continent?

• That might be the moon
• No

Those are clouds, by the way, not water. So what look like islands there are actually mountain summits.

Those are peaks surrounding Targon

Sadly no, however the arctic part of runeterra may be home to beings like "They Who Endure" or "She Who Wanders"..

I'm really late, but one of those islands west of Demacia(most likely the closest to it) might be Senna's home until she became a Sentinel of Light.