Rohan/Rohane and Donovan family connections

Hi, my grandfather, known as John J Donovan (b. ca.1876) here in the states, emigrated from Ireland, arriving in Boston in May of 1894.  His brother Jeremiah (b. 1869) had arrived in 1892, and his sister, Mary (b. 1878) arrived in either 1894 or 1898.  On the marriage records for all three siblings in Boston were listed parents Bartholomew Donovan and Hanora Calnan.  A member  of another Donovan immigrant family, however, told my parents that these three were not Donovans at all, but were named Rohan or Rohane, and that they changed their surnames when they arrived in Boston.  I have found a marriage record of Batt Donovan and Hanora Calnan/Calanane in 1866 and no records of their children. However, I have found records that seem to be of the siblings' births (and two other brothers, William and Timothy) in Cork as "Rohane," with Bartholomew Rohane as father and Mary Honora or Honora Calnan (Calanane, etc) as their mother. There is a grave in the old Templetrine Graveyard with Bartholomew, Hanora, William and Timothy Rohane.

  I have read of the existence of "double-barrelled surnames", where to a clan name (like Donovan) was added a descriptive name (like Rohan/e) and this distinguished families with extremely common names, yet the clan name (in this case, Donovan) was considered more important to pass along to an emigrating family member to keep it alive in their new home. 

Soooo, does anyone have any familiarity with the existence of such double surnames?  Any lore in the Ballincollig, Bandon, or Courcey's area concerning Donovan-Rohan/es?  Or Calnans?

Thanks in advance

Margaret