The problem with Donegal at the end of the day is that it too far from everywhere. Ok, it is near Derry, but that's not a great asset to be honest. Only West Cork is harder to get to from the Dublin area, but West Cork has a good sized city on its doorstep, a city with an economy that isn't based on dole, grants and handouts!
Donegal is beautiful, and in my view well worth a trip anytime, but most people don't see it that way. If you live in the general Dublin area, and let's face it that is where the money is, you can be in Paris quicker that in Donegal if you want to take a break. Tourism will always be just a trickle in Donegal.
Industry has been moving out of the county of late too, the well-publicised drain of industry in the last few years has not been replaced by anything else. When people stop selling houses to each other in Donegal, there won't be an economy worth talking about.
Sadly, I wouldn't rate Donegal -- one of my favourite places -- as an investment destination.