Hello Loralee, a quick answer so you get a notification tonight, there are lots of Fergusons but in the 1901 census there is a boarder in a house, he is a saddler (horses) it is in Church Street, he is 23.
I can find no birth records on the civil recorrds which are online free, to the names you mention, this site shows Harbron in St Marys Drogheda, County Louth but a Church of Ireland (Anglican) there are 3 people mentioned on the headstone and I have the transcripton if you want it. It is in the diocese of Meath. http://jbhall.clahs.ie/
Where did you get the birth certs you mention for Dundalk, I also had a quick look at Roots Ireland but it just went down for planned maintenance.There are a number of Ferguson marriages in Dundalk but nonoe to a Herbron person, see https://www.irishgenealogy.ie/en/ Civil records from 1864 are free for the period you mention, I did notice a Phoebe Ferguson age 61 or so died at some point, not a common name here, what religion are they as some Presbytereian records are difficult to locate. The 1901 and 1911 census are free online.
If you can specify a record I cansee I will dig a bit more.