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There are online resources which will fill in some of Mary's life after her husband died.
Thomas Murtagh's death-
https://civilrecords.irishgenealogy.ie/churchrecords/images/deaths_returns/deaths_1900/05747/4623052.pdfIrish News & Belfast Morning News, 26 Nov.1900: MURTAGH—At his residence, 59 Great Victoria Street, Belfast, Thomas Murtagh, son the late George Murtagh, formerly of Royal Avenue.
Mary's household in 1901-
census.nationalarchives.ie/pages/1901/Antrim/St__George_s_Ward_Belfast/Great_Victoria_Street/955804/
1911- Mary & children living at 1 Paulett Ave.- form filled out incorrectly so we know she's 6 children in total but 2 had died before 1911-
census.nationalarchives.ie/pages/1911/Down/Pottinger__part_of_/Paulett_Avenue/222713/
If this is the same Mary then it looks like son Harold became a doctor-
Murtagh Mary of 213 Mountpottinger Road Belfast widow died 24 May 1948 Probate Belfast 23 December to Harold Murtagh medical doctor. Effects £1451 7s.
Northern Whig, 19 Feb.1935: ... linked Belfast and Bradford families took place at the Catholic Church, Sowerby Bridge (Yorks), the bridegroom being Dr. Harold Murtagh, Ryburn House, ... son of the late Mr. Thomas Murtagh and Mrs. Murtagh, Belfast, and the bride Miss A. Frances ...