Albert,
You?ll see from the 1901 census that Robert was born in Co Antrim (as opposed to Belfast, where his wife and children were born). That plus the fact his father was a farmer, tells you he came from a rural area somewhere in Co Antrim.
Robert was born c 1849 but birth registration did not start in Ireland till 1864 so you won?t get a birth certificate for him. You might find his baptism in church records. Some baptism records are on-line but many are not (especially for Presbyterians, as this family evidently were). I searched on the rootsireland site for a Robert s/o John born c 1849 but none is listed. However as I say, that may just reflect the fact that the relevant records are not on-line rather than that no record actually exists.
A farmer would normally be listed in Griffiths Valuation, so assuming he were still alive in 1861, you would expect to find him to be listed. Unfortunately there are over 20 John McClures listed in Griffiths for Co Antrim. So a bit of a needle in a haystack, to work through all those sets of church records.
http://www.askaboutireland.ie/griffith-valuation/index.xml?action=nameSearch
If you do get a clue as to where he was born, there are copies of most church records in PRONI, Belfast (the public record office). The records are on microfilm and a personal visit is generally required to search them. There are several hundred Presbyterian churches in Co Antrim and so you will need some clues as to Robert?s place of birth in order to narrow that search.
I searched to see if Robert left a will, but there is no trace of one. Or at least if there was one, it wasn?t probated. Difficult to know what to advise you to do next. I cannot immediately think of any easy way of finding Robert?s place of birth. Did he have any siblings that you know of? If so, have you searched censuses to see where they were born? If they died outside Ireland, then in some jurisdictions place of birth is recorded on the death cert (and sometimes on marriage certs too). Did any of them serve in the armed forces? If so, those records may contain a place of birth. If Robert has a gravestone, it might say where he was born. As might an obituary (if there was one). I assume you have the 1875 marriage certificate and it doesn?t contain a rural address for Robert?
Elwyn