TEI:
Welcome to Ireland Reaching Out!
I located the 1879 church marriage record on the subscription site Roots Ireland for David Grennnan/Mary Condron but did not find a civil record which would have been helpful.
Date of Marriage:15-Oct-1879
Parish / District:KILBEGGAN
County:Co. Westmeath
Husband DavidGrennan
Wife MaryCondron
The baptismal records I located on Roots were for the following cchildren all with the mother Mary Ryan: Mary 1880 David 1882 Ann 1884 Elizabeth 1885 Josephine 1887 Gertrude 1893. They line up somewhat but not exact with the children in the 1901 census record. Below is a link to the 1895 death record for Mary Grennan which does not show a maiden name. I also found an 1896 death record for another Mary Grennan who was likely a sister to David Jr.
https://civilrecords.irishgenealogy.ie/churchrecords/images/deaths_returns/deaths_1895/05914/4679064.pdf
https://civilrecords.irishgenealogy.ie/churchrecords/images/deaths_returns/deaths_1896/05890/4670995.pdf
Roots Ireland had baptismal record for siblings of David Jr. as follows Mary 1841 (who died in 1896) Anne 1843 Eliza 1855. Big gap between Eliza and Anne. Here is the link to the parish register https://registers.nli.ie/parishes/0943 You may want to scan to see if there are other records which were not transcribed.
I think Mary Condren and Mary Ryan were the same person. Here is the link to the parish register for the October 1879 register https://registers.nli.ie/registers/vtls000635218#page/48/mode/1up You will notice that the female witness to the marriage was Anne Ryan. My guess is that the priest transposed the surnames for the two females and David Grennan actually married Mary Ryan.
Questions?
Roger McDonnell