They can be used from most rooms across the bay to get back to town, though there are couple exceptions. Going from town to the hunting areas, there is at least one room each in the Ruined Tower, Miasmal Forest, Oteska's, and the Keep that can have a ring set in them (one is obvious, the rest may or may not be). This allows instant travel from town to every hunting area the Adventurer's Guild will send you to except the Krolvin Ship and Shattered Moors, neither one of which take long at all to get to from the closest room that rings work both ways in with.

What more do you want?

Starchitin

PS Now that I think about it, I think there might be a room in Shattered Moors that rings work both ways in, but I'd need to test it to confirm.

A severed gnomish hand crawls in on its fingertips and makes a rude gesture before quickly decaying and rotting into dust. A gust of wind quickly scatters the dust.