Hi, Nathaniel.
Have you looked through the records available for free at https://www.irishgenealogy.ie/en/ ? The 1899 marriage record for John and Susan is here : https://civilrecords.irishgenealogy.ie/churchrecords/images/marriage_returns/marriages_1899/10386/5781806.pdf (and gives Susan's father's name as Robert).
You can find John and Susan on the 1901 census at : http://www.census.nationalarchives.ie/pages/1901/Antrim/Clifton/Newport_Street/962449/ (which gives her date and place of birth as about 1878 in Belfast) - and you could check for them on the 1911 census also through http://www.census.nationalarchives.ie/search/.
Once you've got an idea (from the census) of where and when she was born and (from her marriage record) who her father was, you can go back to irishgenealogy to look for a birth record ... and just keep working it back.