Pass just the element names and convert them to tags in your shortcode:
[irt_top_posts post_type="showcase" container="div" number="10"]
In your callback:
$return "<$container>$return</$container>";
Pass just the element names and convert them to tags in your shortcode:
[irt_top_posts post_type="showcase" container="div" number="10"]
In your callback:
$return "<$container>$return</$container>";