Martin is a very highly regarded acoustic guitarist but not too well known for his banjo playing ( I think he plays a lot less banjo these days compared with earlier -the 1970s -in his career.
Indeed, I was talking to Ken Perlman at a gig in the UK a few years back and he was unaware that Martin played the banjo ( Ken was familiar with Martin's guitar work )
Below is a link to a page where listeners can hear Martin's wonderful take on the traditional ballad "The House Carpenter"