The issue was that in the Grails client I did not create a userDetailsService. I implemented a custom class within Grails' services folder (See http://grails-plugins.github.io/grails-spring-security-core/guide/userDetailsService.html):
public class MyUserDetailsService implements UserDetailsService {
// Required methods here
}
then referenced this in resources.groovy:
beans = {
userDetailsService(MyUserDetailsService)
}