Dord is the earlier spelling. I'd imagine the spelling in Scotland still carries an accent (grave) given its derivation. O-u differentiation is not huge and the pronounciation of dord (or more commonly bord for table) varies from long o to short o or u to a diphthong and even long u in various dialects in Ireland today.

In modern lexicography, dord is used for bass instruments or to denote a related instrument of lower pitch as in the case of "dordveidhil" for violincello. The use of dord to denote a deep voice is attested in old sources. The name of the group Dordán is another noun derived from a verb.

Pádraig Mac Piarais' title "Dord Féinne" is based on Dord Fiansa in older sources where dord is a chant or refrain and some of the sources imagine a noise from beating together the shafts of spears accompanying the chant.

As this was in a context of warfare, I'm not sure if that makes it inclusive or exclusive to the varying opinions on guitars and percussion in Irish music sessions!