Hello Lin,
I searched for a marriage of John, and Mary, using lots of variations, with little success.
I think your best bet would be finding the births of young Roger, or Elizabeth.
If you go by the 1881 census i provided the couple are in Blantyre, Lanark, in that census, there is a birth of a Elizabeth Feeney, that spelling 1883. Blantyre, Lanark, which would fit age wise for your Elizabeth, its a gamble maybe worth trying, that birth would provide parents names and should also provide parents date and Place of marriage.
There is also a possible birth for young Roger, 1879. Holytown, Lanark, with the surname coming up as Finlay, could possibly be the register mistakenly writing down Finlay, instead of Feeney, another gamble.
By 1901 census the family are in Bellshill, Lanark, now that is where the death of Roger Feeney, is 1898. although his age is well out how many Roger Feeney's, are there in Scotland at that time well worth a look as it should provide his wifes maiden name and parents provided whoever registered it knew that info, but i would wager a bet his son John Feeney, most likely did.
Regards,
Jonn.