Susan,
A few questions - assuming you are still looking for Michael ...
Is this his death (from FreeBMD) - Michael FORKIN, aged 59, in 1878 in Burton. If so, have you searched for a death/funeral notice in local newspapers (that's how I found the Co Mayo townland my Ggfather came from) or might there be a gravestone with an inscription?
Do you have Mary's maiden surname? If not, the birth certificate for your ancestor James should give it. I can't see the birth of a James in Burton on FreeBMD (which is, admittedly, incomplete). There's a James FORKAN in Sculcoates in the East Riding of Yorkshire in 1854, and a James FORKIN in Walsall in Staffordshire in 1856.
Do you know when Michael and Mary moved to England? Do you have Michael and/or Mary on all possible censuses (including 1841 and 1851 if you don't know when they moved to England)? Perhaps there is a more precise birthplace on one of them (not the 1841 of course).
Do you know whether Michael and Mary married in Ireland or might they have married in England? I notice that there is the marriage of a Michael FORKIN in 1858 in Congleton, Cheshire (on the border of Staffordshire) with a Mary O'FLYN on the same page.
I also notice a marriage of a James FORKIN in 1857 in Burton on Trent - perhaps related to Michael?
Regards,
JAP