the ones in Bog are tough and very hard to solo with fighter(i mean high yellows). they have good resistance against blades ..and prolly against all weapons. Rare drops(and im still talking about the ones in Bog..) , but good af 70 scale stuff for lvl 30+ and some necklaces for mages. Mermen's standing wont change except those mermen in Connla's Well can turn to aggro.
In Shannon Estuary, East of Connla, following the river branch to the east, you should find a water pond inhabited by Mermen. It's also there that Caora the Mother of all Monsters lives, with her offspring.
Other mermen can be found in Lough Gur, but they are much tougher (purple to 23, so they must be at least 28)