I can't find a marriage at the moment (although there is another couple in Westmeath named William Brien and Mary Glynn who married in 1863 - but this couple were having children in Westmeath at the same time as your William and Mary - so that's just a coincidence!).
I am not sure if you are aware of another 2 children born to your couple, as you have only mentioned 2 -- but parish registers show;
Margaret O'Brien 1876
Mary Brien 1878
Catherine Brien 1880
Ellen Brian 1882
All born Moycollop.
William appears to be a Farm or Land Steward.