How can I allow an external application to connect directly to my Ruby on Rails-integrated PostgreSQL database?

I have one database, and several applications that need to use it (one Ruby on Rails app, with which the database is integrated by default, and a bunch of Minecraft servers running Java plugins). Right now, I am using my web application's REST api to access the database.

So: plugin -> api -> db -> api -> plugin

I am looking for a way for my plugins to access the database directly, without having to first go through the web application.

plugin -> db -> plugin

How can I allow an external application to access my database? How can I ensure that only certain applications are allowed to access the db? (Also: SHOULD I be doing this at all?)