Marketers and salesmen will often tell tales about how particular patterns in Aran sweaters were once used to identify the bodies of lost fisherman. However, this is a myth. From wikipedia:

It is sometimes said that each fisherman (or his family) had a jumper with a unique design, so that if he drowned and was found, maybe weeks later, on the beach, his body could be identified. This misconception may have originated with J.M. Synge’s 1904 play Riders to the Sea, in which the body of a dead fisherman is identified by the hand-knitted stitches on one of his garments. However, even in the play, there is no reference to any decorative or Aran-type pattern. The garment referred to is a plain stocking and it is identified by the number of stitches, the quote being “it’s the second one of the third pair I knitted, and I put up three score stitches, and I dropped four of them”. There is no record of any such event ever having taken place, nor is there any evidence to support there being a systematic tradition of family patterns.

Kate Davies, writer and knitter, explores this a bit more in an interesting piece she wrote on needled:

While Gahan encouraged the talented knitters of rural Ireland in their creation of elaborate báinín ganseys, Ó Síocháin invented myths of ancient origin for the sweaters in his publications about the Aran Islands. In his book Aran: Islands of Legend , for example, Ó Síocháin footnotes the misleading idea that “the Aran gansey has always been an unfailing source of identification of Islandmen lost at sea” with a reference to his own company “full particulars regarding the handcraft products of the Islands can be obtained from Galway Bay Products, Ltd.”