Hi Kathleen Thanks for your very prompt response to my post.
As you suggest I do have the marriage certificate for Michael C and Mary Coyle -12 Oct 1857 at the RC Chapel in Felling, Gateshead, Co. Durham. This shows M's father as James Convey deceased (jobbing labourer) and Mary's father as Peter Coyle, chemical labourer (almost certainly at the chemical works in Felling Shore at that time). The cert. also gives 2 witnesses; Stephen Timlin & Ann Doud; clearly Irish names and from the local community in Gateshead. I have found a reference to a Stephen Timlin in Mayo in reference to properties in Attymoss & Crossmolina parishes in Co. Mayo. In 1876 he was 40 which puts his birthdate around 1835/6 in common with Michael.
I've seen the LDS FamilySearch data and the relevant UK census returns but they all give a very generic 'Ireland' as place of birth.
What I'm hoping for is a clue to point me in the right direction to follow up on James Convey and Peter Coyle in Ireland. Asyou rightly point out this isn't helped by the fact that Convey can also be Convay, Convoy, Conway depending on the scribe or transcriber and that's before we even get to records in Latin!