Haven't been in the BDM office this week, but had a quick look for your MARGARET LENNOX while at the National Archives.
I checked the 1901 Census, and the Thoms Directories for 1889 +/- 3 years either side, but no Lennox family living at either Harolds Cross Road or Cottages. Can you have another look at Catherine's baptism cert and see if it says anything more specific than Harolds Cross (which was and is an area, rather than a street name)?
Also, there doesn't appear to any such place in Co. Dublin called 8 Bank Side (as per Margaret's father's place of residence at the time of her marriage). Any further info?
Look forward to hearing from you.
Kind regards, Colette