Ireland in late August/ September

Rabbies and Paddywagon do tours. Also once you have identified places you want to stop, do a google search for day tours from .... to pull up more ideas.

If you are in Northern Ireland, you might want to visit the Ulster American Folk Park near Omagh which tells the story of some of the families who emigrated from Ulster to America and how they got on in their new life there. There are reconstructed buildings from across Ulster as well as a reconstruction of part of the ship that they would have sailed on (families had a 6'by6' living space and conditions were grim. They also didn't know exactly how long the voyage would take or where they might land. It all depended on the wind, weather and tides.) There are also buildings from the States. Costumed interpreters will tell you the stories of the different families.

You can travel between Dublin and Liverpool by plane or ferry. It's generally easier to fly in/out Dublin from the US.
Day tours should generally be booked ahead. You might also want to make an appointment with a genealogist at EPIC in Dublin, but I recommend finding out as much as possible about your family before you go so that you can plan to visit the places they came from. I see that there is a direct bus from Dublin to Enniskillen. These long-distance buses are quite comfortable, in my experience, and of course, they offer great views of the countryside along the way. With a town of that size that isn't likely to be covered by the guidebooks, I generally use Google Maps to locate hotels/B&Bs close to the city center. It looks like taxi service is available there, so getting around town should be relatively easy. County Longford, on the other hand, could be more of a challenge. Depending on what part of the county your family came from, maybe Athlone would be a good base? There are sights to see in Athlone itself, including a castle and a very old pub (maybe the oldest in Ireland?). In my experience, these "heritage" sights are more meaningful than some of the more touristic options when visiting Ireland.

You can travel between Dublin and Liverpool by plane or ferry.

The Dublin to Liverpool passenger ferry service ended a number of years ago- now it is freight only.

Normally you have to go to Holyhead for Dublin as a passenger (foot or driver). While Holyhead port is closed there is a temporary vehicle only passenger service from Liverpool. Stena currently intend to return to Holyhead on 16 January, Irish Ferries are not expecting to return there for a number of months and are running a temporary Fishguard to Dublin service (little use from Liverpool).

Thus it is either flight on Ryanair or train to Holyhead (after the port re-opens) for the ferry.

The complication is that Enniskillen is in Northern Ireland so records are at PRONI in Belfast. County Longford records will be in Dublin, so far as they survived the Four Courts fire of 1922 and the other prior problems of census loss and poor civil registration in the early years.
The Liverpool to Belfast passenger ferry still runs, or flights on Easyjet.

Thanks, isn32c! I knew there was a ferry to Liverpool, just didn't realize it was from Belfast, not Dublin. You're the expert on all things transport-related in Ireland, for sure.