I have a feeling that in addition to the 'Location', 'Marriage/Residence' issues, there may also be one relating to loss of 'Names'. I say this because I can't get 'the other party' to a marriage (that I am confident took place) to show up in any of the Family Search Marriage Indexes for Ireland. I have taken a chance by ordering some birth certificates for two boys that I think are her children, as well as a marriage certificate for the man I think she married around 1877.
(Looks like there is a bit of a backlog again at Roscommon)
If my guess is correct, then there are quite a few names missing - at least from the marriage index. If you input Teresa McNally as your search term into Record Search Pilot you get 110 results. Refining it to Marriage/Residence reduces that number to 22 - look at the number of marriages that took place up to 1900 for Teresa McNally's and reduces to
only 1, all the others take place after 1900. Seems strange, or was Teresa not a popular name around the time of her birth in about 1848?
Hopefully some of my issues might be cleared up when the 1901 census of Ireland comes on line later in June - fingers crossed.

As an off-the-wall suggestion I wonder if the LDS would look at offering FreeBMD the opportunity of hosting the Irish BMD Index on part of the FreeBMD site? After all the data fields are the same for both indexes and we would then have the benefit of being able to see the the potential 'other parties to marriage' before ordering a speculative marriage certificate! The precident has already been established because FamilySearch is now collaborating with other genealogical online providers such as FindMyPast for the 1841 and 1861 census of England & wales.
I wonder if FreeBMD would be open to such an approach?
Just a thought.

H