You can use \Phalcon\Mvc\View getRender() method:

public string getRender (string $controllerName, string $actionName, [array $params], [mixed $configCallback])

Pro tip: $controllerName can be empty or used just as additional catalogue name. It is possible to make your custom rendering function as follows:

public function myCustomRender($params = array())
{
    $currentViewsDir = $this->view->getViewsDir();
    $this->view->setViewsDir($this->getMyCustomViewPath());

    $content = $this->view->getRender('', $this->myCustomTemplateName, $params);

    //rollback viewsDir
    $this->view->setViewsDir($currentViewsDir);

    return $content;
}