Seriously. I know this sounds crazy. Here's the deal. Fly in to Dublin 5:15 a.m. on a Sunday in early September. Do the HOHO in Dublin, stay overnight. Pick up the car at the airport about 8 am. Spend 3 hours at Bru na Boinne and keep going north.
We want to spend a bit of time in and near Glenarm but there's only one B&B there -- ancestors from Glenarm and Ahoghill. We'll likely stay 2 nights.
Option 2: 1 or 2 nights Glenarm and visit Giant's Causeway on the way to Enniskillen.
Option 3a and 3b: Either of the above, but only stopping in Glenarm and staying a little further north.
Option 4: Your better idea.
Airport to Glenarm without stops is about 3 hours (7 including Bru na Boinne and dinner). Glenarm to Enniskillen via Giant's Causeway is about 4 hours (again 7 including 2 at the Causeway and dinner). Glenarm to Enniskillen via Dungannon is about 3 hours.