Black flag

Happy St Patrick’s Day!  Rather amusingly I’m getting more Irish, more Jewish and more Scandinavian by the minute, according to my Ancestry DNA profile. They keep updating it. 
I saw this black flag with a metal hand above it today. It was just in someone’s garden. I’ve looked it up but I still couldn’t find a definitive answer of what it means.  Lovely sunshine today and it felt warmer.