Thank you so much for your helpful response.
All the children mentioned in the 1891 census were recorded as being born in Port Glasgow. Patrick, James, Mary, John, Joseph and Maggie. The dates of birth for Partick and James match the information you provided. Although I haven't yet been able to find birth certificates for the 2 boys I did manage to find one for Sarah in 1872 registered in Greenock rather than Port Glasgow. Sadly it appears that she died at a young age. The certificate shows that John and Catherine both Catholic, married in Port Glasgow in 1869 but I can find no trace of them on Scotlands People as yet.
Once again very many thanks for your kind assistance and advice