Filter `'login_redirect'`.

Sample code, not tested:

    add_filter( 'login_redirect', 'wpse_78150_login_redirect' );
    
    function wpse_78150_login_redirect( $url )
    {
        # uncomment the next line for debugging
        # var_dump( wp_get_current_user() ); exit;
    
        if ( ! current_user_can( 'custom_role' ) )
            return $url;
    
        return admin_url();
    }