Similarity to Memories of Murder
This film is similar to Memories of Murder in a lot of ways.
Both are set in the 1980s with the background of recent political and social upheaval, in small farming towns/communities (with very similar scenery). Both films pair two mismatched detectives (one rough and ready, somewhat brutish, with another more cerebral 'city slicker' type). Both involve dead women, where there could be a pattern for serial murder with a somewhat unexplained motivations.
It's been a while since I saw MOM but Marshland just seemed very very similar.