John Sweeney Mary Donovan

I have a brick wall for my ancestors, I have John Sweeney born Mayo from census records he married Mary Donovan in Fermoy 26th of April 1874 on the marriage certificate his father is listed as Patrick Sweeney Labourer and Mary Donovans father is listed as Gardener John Donovan. Mary Sweeney nee Donovan on census records is listed a birth place Cork Ireland. John Sweeney was in army for many years and went to India his son John sweeney is listed as being born India 1876 and daughter born off Spanish coast in 1878 this was Mary sweeney, however one daughter Bridget is listed as being born 26th Feb 1875 Ireland. They then appear on census record in Haslingden Lancashire in 1881 with more children born in Haslingden then Darwen Lancashire. On one census record it has a nephew living with them in Lancashire by the name of John Mitchell but I cannot find where this person fits into the family line. 

I cannot find records for either John sweeney mother or Mary Donovans mother nor marriage certificates for either John or Marys parents. John sweeney had a brother James 1854 listed on some records as born India on others i.e. census 1881 born england  his brother married Minnie Mara and they had a daughter Catherine who were living with John sweeney on the 1881 census. 

I am trying to find out john sweeneys parents to trace back further and also Mary donovans parents if anybody can help? thanks