Conor McGregor: Ireland's ultimate fighter
16 July 2014 Last updated at 10:06 BST
You may never have heard of it, but mixed martial arts is said to be the fastest growing sport in the world.
This weekend some of its best-known fighters, including rising star Conor McGregor, will compete at a major event in Dublin.
The Ultimate Fighting Championship regularly attracts sell-out crowds but some doctors have raised serious concerns.
Chris Buckler reports.