The Aran sweater is seen as an intrinsic part of Irish culture and history. The sweater comes from the Aran Island that is just off the west coast of the country. They are a rugged place and the only consistent food source was the sheep that grazed on the scrabby grass that could barely support any crops and the fish in the sea around them. The sheep were too useful as they provided the wool to make the sweaters. The wool was a special merino type that was water resistant and kept the men warm. You can get one of these incredibly useful items of clothing from Shamrock Gift.
The women of the island wanted to distingh their clothing from that of the people of Guernsey who had shown them how to do it. They worked in ancient Irish designs that harped back the Celtic knots of the pagan past. This gave rise to the myth that the sweaters were some kind of ancient skill that was passed down from the past. In fact they came to the islands at the end of the 19th Century.
One way the myth was strengthened was by the work of John Millington Synge. He wrote a story called Riders to the Sea which is set on the main Isle of Aran. The story is about hardy people who battle the sea and refuse to be crushed by its relentlessness. In the story a fisherman is lost and when he is washed up on the shore he is identified purely by the stitches in the sweater and the individual nature of it.