I was led to believe that a geansaidh (Guernsey) was specifically a fishermen's jumper, which was hand knitted. Each man wore a jumper with a different pattern. The reason appears to have been a tad morbid: if they went into the sea and their bodies subsequently recovered, they'd be recognised by their Guernsey...
That’s more folklore than truth; they didn’t really do that. It’s true that very few guernseys were exactly alike, but different stitch patterns and styles were characteristic of different regions, Fife, Aberdeen, Mousehole, and so on. It’s not that anyone made sure each man on a boat had a different one. It’s just that no patterns were written down like they are today, so knitters arranged stitches in their own way. Q.v. Gladys Thompson, Patterns for Guernseys, Jerseys, and Arans or Rae Compton, The Complete Book of Traditional Guernsey and Jersey Knitting.