A few random links with info about the myth.
http://www.irelandseye.com/paddy3/preview.htm
http://en.wikipedia.org/wiki/Dullahan
http://www.monstropedia.org/monster/Dullahan
http://www.irelandseye.com/paddy3/preview.htm
http://en.wikipedia.org/wiki/Dullahan
http://www.monstropedia.org/monster/Dullahan