Ó Deargáin is a Gaelic-Irish surname, which was found in Leinster and Munster.
It is now anglicised as "Dergan", "Dargan" or "Dorgan".
See also Michael Dargan (1929–2023), Irish cricketer. William Dargan (1799–1867), the father of Irish railways. Theo Dorgan (born 1953), Irish poet, writer and lecturer.
References http://www.irishtimes.com/ancestor/surname/index.cfm?fuseaction=Go.&UserID=