They are $2.50 and SO yummy. A description of the burger: "Their Beef Slider is made from in-house ground chuck, short rib and sirloin and served with caramelized Vidalia onions." -gothamist.com
Reviews and info: http://www.yelp.com/biz/mark-new-york
Oh and you can have a Guinness shake, if you really want to have a good time.