Dord/durd comes from the conventions of Irish spelling. Dord would be correct for something rhyming with 'word' in the modern caighdeán, but durd might have been used in the 19th century - for instance, An Bonnán Buí, The Yellow Bittern, is An Bunnán Buí in 19th- and early-20th-century versions.

Similarly, what was always 'bas' when I was a child - the palm of the hand, pronounced to rhyme with 'moss', is now almost always written 'bos', which should properly be pronounced to rhyme with the English word 'bus'.