McNickle/Dampsey County Derry 1870s

[Cedas]

Has anyone come across any McNickles (Mcnicolls or any spelling variation) and Dempsey or Dampsey?  I am stuck with a couple who are in Liverpool 1881 and acc census came from Co Derry. Joh Mcnickle(or any variation) and Mary Dempsey had a child in Liverpool in 1877 but I cannot find any evidence of previous children or of a marriage in Liverpool.

They were (acc my 95 yr old gt aunt) catholics. John McNickle(McNcicol etc) was a boot maker/shoemaker according to Liverpool census and trade directories and my gt grandmother's birth cert.
He must have learned his trade somewhere. Possibly in Co Derry?
 My gt gradmother's birth cert tells me her mother was Mary McNicol formerly Dempsey. (Illiterate sigend with X)

Her birth cert is the earliest I can find them

any suggestions appreciated.