Example function to load plugins within wp-content/themes/mytheme/plugins/

functions.php

/**
 * Add in-theme plugins
 */
function my_theme_plugins() {

	$plugins_to_include = array(
		'wordpress-fieldmanager/fieldmanager.php',
	);

	if ( count( $plugins_to_include ) ) {

		$theme_plugin_dir = get_stylesheet_directory() . '/plugins/';

		foreach ( $plugins_to_include as $plugin ) {

			if ( file_exists( $theme_plugin_dir . $plugin ) ) {
				include_once( $theme_plugin_dir . $plugin );
			}
		}
	}
}
add_action( 'after_setup_theme', 'my_theme_plugins' );