As you will travel to Northern Ireland, have you maybe looked at renting from Northern Ireland instead of Dublin?
There might be more options there, and you could easily take the bus from Dublin to Belfast directly from the airport.
It does add time to your journey, but it might be an option worth checking.
Some accommodations may also offer free cancelation or be flexible with the dates of your stay if you decide to do part of your journey via public transport. I use Rome2Rio.com
when planning trips around the island as it lists the national and private services in one place.
What part of Donegal are you planning to visit? I spent last weekend there, one of my favorite places on the island! 💚