The Emigrant Thing
It is most common for those leaving a region like Co. Longford to follow either relatives, friends, or others from the same local community.
There are a variety of ways to determine who those connected individuals and families might be in the new lands, but the real clincher is when someone marries not long after they have settled in the new country. If you then find the home location for the spouse, very often the home for the person of interest is in the same parish, or in one of the parishes immediately around that of the first person.
So find the other person's location and then the person of interest can be found.
At times the following of one emigrant by another can take place over 20 or 30 years. So keep an eye on those who settle nearby in farming communities.
In the case of the Moffatt origin mystery, Lt. Colonel Bob Moffatt, who lived in the 1980s in Ottawa, knew of the strong connection between Moffatts and Walshes, with two marriages taking place within a few years after emigration. He learned of a Walsh Reunion to take place, and invited himself along.
There, he met Gwen Richards, formerly of Delaware Ave. in Ottawa, who had a list handed down to her of all the children of Samuel Moffatt, together with their birth dates. She assumes it came from a family bible that has long since disappeared.
She gave a copy to Bob Moffatt, and he handed a copy to me. When we found an exact match in the Killashee Parish records, the connection was solidly made, not just for the eldest child born in 1815, but for the parents.
Following the Walshes led to solving the Moffatt mystery.