Thinking back to my golfing adventures in Donegal I would suggest -
The Sand Hills at Rosapenna is the most challenging course golf wise. Over and through big dunes most of the time with numerous elevated greens, some of which have been altered not necesarily for the better (eg 13th, 18th). Sandy Hills is probably something of a marmite course - "you either like it or you hate it" - I happen to like the course.
The OTM at Rosapenna has a delightful, flowing, apparently mainly Harry Colt back-9 with a thoroughly nice, but perhaps somewhat less memorable, front-9.
The Old at Ballyliffin is an olden day type of humps and bumps and hollows 'ground game' style course - "made by 1 man with a mower" as a local described it to me.
The Glashedy at Ballyliffin is a 'made by big machines' course. Nice routing up/down the big dune but graded fairways with limited, if any, micro-contouring.
If I had 3 rounds I'd probably play the Sandy Hills and the OTM-18 at Rosapenna plus the Old at Ballyliffin. If I had time for 2 and a half rounds it would be Sandy Hills plus the 2nd-9 of the OTM and the Old at Ballyliffin. But then again I'm rather partial to Portsalon, Narin & Portnoo and delightful Dunfanaghy so I might be inclined to venture to one or more of them instead.
It is worth considering that the drive from Rosapenna to Ballyliffin or vice versa is at least 2 hrs (and probably a bit more if you go via Portsalon GC).
Atb