To help search for you can you clarify
Where did Mary's year of birth come from?
Could this be out by a year or two?
If so, there are at least 10 births of a Mary Field or Mary Fields in the Dublin area between 1870 and 1874
If Thomas Butler was in the army have you located his military record?
This may refer to his marrying and where; more detail on Thomas would allow us to look
e.g. DoB, regiment...
Soldiers required the permission of their commanding officer to marry; if this was not obtained a certified letter from a church was deemed null and void and many times the couple would have to "marry again"!
Also what denomination were they?
If Mary left Ireland at age 15/16 that would have been 1887/1888 approx.
She would have been underage to marry.
There is a marriage of a Mary Field to a Henry Butler in Dublin in 1893 could this be them?
Are they on UK census on other years?
and especially in 1911, does this refer to years married?
If her father was an undertaker, bear in mid in the 1870s this may mean he was a carpenter, cabinet maker, upholsterer, etc. references to him may be to his trade; he would have to have been the owner of the business to show up on directories of the time.
Field is a bad name for searching for obvious reasons, so the more information the better for searching