No mention of barriers in the man page, I'm also getting confused. is device mapper used for software raid - i.e. /dev/mdX?
Nope. Software raid is the md layer. Nothing to do with dm. Two separate layers although they share a bit of stuff.
If so what are the implications of barriers and where are they turned on / off?
Barriers allow one to ensure true fsync/fsyncdata when used with hard disks that have their write cache enabled. This is not talking about hard drives connected to hardware raid controllers which is a different ball game.
Forgive me for potential off topic, but I too run xfs on lvm which uses mapper.......risky??
IF you are not using hardware raid with bbu cache and of course, if you have disabled the write caches on the hard drives connected to the raid via the raid controller.