States trust me it is a myth. It is the probably the product of an overactive Irish imagination and love for the tall tale. Inhabitants of Western Ireland do tend to have darker hair than E. Ireland but the complexion is usually pale and eyes light. There are exceptions of course.
http://www.darkfiber.com/blackirishMany Irish had dark hair etc long before the Spanish Armada in 1588. Genetic information points to this trait being from the original inhabitants of Ireland around 8000-9000 years ago long before the Celtic invasions.