I suspect that Bailieborough is just the postal town, rather than the exact place where the Muldoon family lived. So a letter to the Muldoon family would be addressed to them with their townland plus Bailieborough. (Bailieborough itself is in a different parish). Given your uncertainty, I would say you really need to get that birth cert to establish exactly where they resided, to have any chance of locating the family home. There are 67 townlands in the parish of Knockbride. You need to be able to narrow your search.
You can order a photocopy from GRO Roscommon for ?4 per certificate. Put the place, year, quarter (where there is one), volume & page number on the application form (anywhere). Don?t worry about leaving some boxes blank. You don?t need to fill them all in if you have the reference details. http://www.groireland.ie/ You have to post or fax the form to them but they will e-mail the copy certificate to you if you wish. Tick the relevant box on the form.
Alternatively you can probably buy a transcript of the birth certificate on the rootsireland site.
Once you have the townland from the birth certificate, go to Griffiths Valuation and see if the family are listed. (If you have difficulty with that, let me know and I?ll do it for you).
Regarding passenger lists to the USA, no records were ever kept of people departing Ireland. Passenger lists were created for people travelling to the US, but were given to the US immigration authorities on arrival. You should check the Castle Gardens and Ellis islands sites (both free). There are also passenger lists on Ancestry and findmypast (both pay to view). Be aware that Susannah may not have sailed directly from Ireland to the US. By far the busiest port of departure was Liverpool (with most shipping agents throwing in passage to Liverpool free, as part of the fare). Some immigrants to the US also went via Canada as it was sometimes cheaper, so don?t exclude that routing.
I have attached a link to a site with information about emigration from Ireland in the mid 1800s. It happens to be for Co Donegal but everything in the link would be equally applicable to Co Cavan:
http://www.finnvalley.ie/history/emigration/index.html