Have you got her death certificate, or any death notices, did they put her death in a paper it could say what townland she came from.
If Martin was in the army he would have had a army pension, his records might say were Margaret came from.
If Martin was RC and joined the Army and then married a protestant he would possible be disowned by his relatives, so would Margaret
How old was Martin?
You could try putting a note in some of the local papers around co cavan
My late grandma ran away with my late grandad and was not aloud to go back home that was in 1905 from townland Rathmore county donegal.