Hi Bob, got engrossed in the news last night but done some searching, the Mary Wiite I gave you appears the most likely, checked Roots Ireland and found a few more in Louth but not near Blackrock, see on the Registers site that that Haggardstown is another name for Blackrock parish, you can see the baptism entry here https://registers.nli.ie/ She has a number of siblings on Roots Ireland, some address is given as Dublin Road, Haggardstown, if it is the same Dublin Road as now it is a mile or so from Blackrock Village you can see new and old maps here https://www.osi.ie/ There were a number of siblings, born from 1863 to 1876.
In 1854 you can see a number of White families living in Haggardstown, you can manipulate the map icon to find the plot, tricky but doable. http://www.askaboutireland.ie/griffith-valuation/
There are a number of Ryan families in Louth at this time also, the two nearest are in Dromiskin parish, did not really find a birth of a Frank or Matthew to suit though.
There is an interesting site on Louth here, you may find some grave references and if you do let me know as I have hard copy of a lot of them but do not think there is one on Blackrock, the Whites were tenants of Lord Clermont in Griffiths Valuation. http://www.jbhall.clahs.ie/
There are one or two old pictures of Blackrock in the National Library site, check digital photos and Louth or Blackrock, Louth, there are at least 2 other Blackrocks in Ireland. https://www.nli.ie/digital-photographs.aspx
There are some Louth Genealogy Facebook pages and old photos etc if interested too.
Found a lot of Matthew Ryan births and a few Frank but none jumped out in like she married a neighbours child, of course at this point travel was much easier, trains everywhere.
I live about 20 miles from Blackrock so if anything comes to mind please revert but we are locked down for at least 4 weeks and maybe 6.
Regards
Pat