There is this death in Dublin South, which fits for Harolds Cross, but he would have been only 13 in 1910, for the year of your proposed marriage for him
John O'Donoghue
Date of Registration: Apr-May-Jun 1957 Death Age: 60
Registration district: Dublin South Volume: 2 Page: 347 FHL Film Number: 257856
Could the cemetery say what age was claimed for him?
When do you think John and Margaret were born, and when about for that marriage - a few years after 1910?