Traditional genealogy of O'Duibhdhiorma (simplified version) showing descent from Prince Eoghain of the Northern Ui Neill. Y chromosome DNA test results of several DeArmond surname variant men compared with surnames from other clans of the Northern Ui Neill and Cenel Eoghain, confirm the essence of the genealogies.
A dotted letter "b" becomes a short v and the dotted letter "d" becomes a short y. This produces "Doove Yiorma". When the two words were spoken together as a surname, there was an elision or dropping of the "ve" and the pronunciation becomes "Doo-Year-ma". Dooyearma is how the early Anglo/Irish recording officials first transliterated or Anglicized Duibhdhiorma. On occasion "Dooyiorma" or even "Dughierma" spellings were used. The pronunciations were all similar.
Gaelic Duibh is an intensive adjective meaning very black (hair color). Dubh with the letter "i" missing, meaning merely dark hair and is commonly found in many Irish and Scottish Gaelic surnames such as in Dugh-again or Dugan. D(h)iorma means a member of an armed group of men.
Properly speaking, Ua Duibhdhiorma is the surname of an individual male descendant or clan member. Although only occasionally found, Ui Duibhdhiormaigh is the name of the clan itself. The patronymic prefix Ua is pronounced O' as in O'Keefe. Ui is the patronymic of a clan or clan group as in Ui Neill, pronounced "Eee Neal". Ua has long since fallen out of use in favor of the Anglicized O'. Ui is still used for larger clan groups. A single clan or sept is generally referred to today by the singular manner such as O'Duibhdhiorma. The older genealogies translated from Seanchlo Gaelic into English often don't show that a consonant is lenited. Therefore you will find Duv Dirma, which is actually Duibh Dhiorma.
In various other posts of my blog, I detail how O'Duibhdhiorma over time became DeArmond, McDermott, DeYarman and an untold number of other variants.