The prophet of the captivity, Daniel, bore in his name an amplification of that of Dan (a judge).  The termination signified God the judge, and the alias Belteshazzar, imposed upon him by the Chaldean monarch, is considered to translate and heathenize the name, making Bel the judge.  It is observable that Daniel never calls himself thus, though he gives these heathen titles to his three companions. (Yonge)

According to a note in the Annals of Ireland it would seem that the Irish name Domhnall (pronounced Don-all), whence O'Donnell, has sometimes been Anglicised to Daniel. (Charnock)