Father of John Duignan / Degnen b.1818 County Cavan?

Hi.  I'm trying to identify the biological father of my GG Grandafther, John Duignan b.1818 Cavan.  What I know is as follows:

1. The 1841 Census records John Duignan (Degnen) living in House 1 Drumcoghill Lower with his mother (Elinor), two younger siblings (James and Mary Ann), and his step-father (James Bigger).  The census further records John as being 23 at the time and his profession was a land surveyor, as was the profession of his step-father. 
2. The 1841 Census also records that Elinor's first marriage was in 1816 (i.e. which aligns with John's birth in 1818, presumably their first child) and that Elinor is a native of Cavan, however I do not know her maiden name nor the location of her 1816 marriage.
3. Whilst Elinor and her children were living in Drumcoghill Lower (County Cavan) in 1841, I do not know whether they were living in the area when with her first husband.  I did, however, have a record that John was from Ballyjamesduff (County Cavan), however I have not been able to relocate my original source.
4. The first born son for the 3 subsequent generations were all named John so, potentially, John's biological father may also have been a John.
5. You can find a record of my John Duignan ancestor on the Killashandra (Cavan) page.

Any assistance is much appreciated.

Wayne Duignan.