....
class CustomLdapAuthenticationProvider extends LdapAuthenticationProvider {

def ldapAuthProvider
def daoAuthenticationProvider
def springSecurityService
def userDetailsService
def dataSource
def grailsApplication

CustomLdapAuthenticationProvider(LdapAuthenticationProvider ldapAuthenticationProvider) {
    super(ldapAuthenticationProvider.authenticator, ldapAuthenticationProvider.authoritiesPopulator)
}

@Override
public Authentication authenticate(Authentication authentication) throws AuthenticationException {
    String username = authentication.principal?.toString()?.toLowerCase()
    String password = authentication.credentials
    Boolean isExistingLdapUser = User.withCriteria {
                        eq("personUsername", username)
                        eq("personIsldap", true)
                    }[0] as Boolean
                    if (isExistingLdapUser) {
                        authentication = ldapAuthProvider.authenticate(authentication)
                        springSecurityService.loggedIn ? Person.findByPersonUsername(authentication.principal.username) : null
                        .....
                        .....
                        .....

}
}

@Override
@Transactional
public Authentication authenticate(Authentication authentication) throws AuthenticationException {