Hi there,
You've chosen well, you could do no better than Donegal!
I haven't stayed in Harvey's Point myself but it has a great reputation, sheer luxury. My only objection to it is its location, I think the really beautiful parts of Donegal are further up, on the west or northern coasts, you can't beat the Atlantic. But if the hotel, rather than the location, is more important to you then Harvey's Point should be perfect.
Others worth considering (ie the main luxury hotels in Donegal) -
Castle Grove Country House, Ballymaleel, Letterkenny
Sand House Hotel, Rossnowlagh Beach, Rossnowlagh
An Chuirt Hotel (The Gweedore Court Hotel)
Coxtown Manor, Laghey
Rathmullen House, Lough Swilly
You'll get reviews of most of them here (and most have their own website if you want to see photos or get special offers) -
(but be wary, I'm suspicious about a lot of the reviews on Trip Advisor, it's too easy for owners to submit glowing reviews....and rivals to put in stinkers!).
The Sand House Hotel and An Chuirt Hotel probably have the finest locations, both on the sea. As far as I can work out An Chuirt is nearest the airport in Donegal (Carrickfinn) if you are considering flying (Aer Arran) - definitely worth thinking about, you're there in an hour, rather than taking on a five hour drive.
This map will give you an idea of the location of most of the hotels -