Where is the nearest place to rent a car in Enniskillen? Or is it better to rent a car in Dublin and leave it near Enniskillen?
I've not rented a car around Enniskillen so can't advise on that, but will note that I think it's unlikely if you rent a car in Dublin that they'd let you leave it across the border in Northern Ireland without some significant additional charges.
You could rent a car in Lisnaskea at the Drivalia Car rental company (Monaghan Brothers garage). It's a 20 minute bus drive from Enniskillen.
https://drivalia.co.uk/locations/car-hire-enniskillen/
https://maps.app.goo.gl/YnFm1CxCaLhtGqng8