The garment worn by the women in the picture is called an arisaid. It is an outer garment that normally extends from the ankles to the neck but can be extended as a head covering in inclement weather. It is essentially an unbelted plaid worn over a petticoat.
According to Burt's Letters (circa 1830), the material that it is made from was usually coarse woolen cloth woven in a tartan sett. However, it could also be made of fine worsted or silk. It reaches to the waist behind; one corner falls as low as the ankle on one side and the other part in folds, hangs down from the opposite arm."