A Mulligan family

[LorraineJ]

I have been researching my family tree for a while now and have encountered a monumental brick wall.

My great great grandparents married in Oldham, England in 1848 and in the 1851 census state that they were both from Sligo.  Their names were Anthony and Ann Mulligan and on their marriage certificate there is a Catherine Hart (Ann's sister presumably).  Ann's father is named as Cornelius Hart, and Anthony's father as Thomas Mulligan.  If the 1841 English census is to be believed, I found Ann Hart living in Lancashire in 1841 and I looked for a Cornelius Hart and found him with his family in London, with Catherine showing as a daughter.  Is 1800 too early to get an information on either on their fathers on line?  Forgot to mention that both Thomas Mulligan and Cornelius Hart are recorded as being from Ireland, I presume from Sligo but am not certain on that.

MTIA.