How the structure is?
Where do you add the postgres driver?
The postgres driver actually is a gem. I include it in my ruby gemfile:
platforms :jruby do
gem 'activerecord-jdbc-adapter', require: false
gem 'activerecord-jdbcsqlite3-adapter', require: false
Other than that, I do not specifically include the postgres driver anywhere. I only specify using it in my database.yml (where the database connection is in, e.g. username, password, dbname etc.).
If you start JBoss without your applciation do you see any message that the postgres driver is loaded?
Do you copy the postgres driver jar into deployments? Maybe with different jar names?
I'm not familiar with JRuby