I noticed this fascinating thread because my twin brother in Ireland is also a John F Duignan!
This must be one of the most vexatious names to English ears. The variety of spellings and pronunciations I have met all my life is legion. Etymologically the name derives from two Irish words, dubh (black) and ceann (head), making it mean "Descendant of the Dark Haired One," a sort of Irish Schwarzkopt!
Where I grew up in Co Longford it was universally pronounced 'Degnan' and even sometimes spelled so, which used leave me difficulties when I tried to cash a cheque ('check' to Americans!) My pupils in school often gave me 'Mr Daglan' among other variants, and my late wife was intrigued by how overnight shebecame 'Mrs Dagenham'! My coup de grace came on a typewritten response to a hand-scrawled note from me and began "Dear Mr Dungman." After that I was more careful!
Three of my uncles were priests in USA. All of them re-added the ancient 'O', making it 'O'Duignan" and pronouncing it 'O'Dignan.' When I first visited Uncle Frank's famous shrine to St Patrick in the village of St Patrick, MO, I was asked why had "dropped" the 'O'! I replied that the 'O' had been lost in Ireland for a very long time and re-added by my uncles.