Just to give this a push, how much driving / hotel switching is acceptable?
If lots, then fly into Knock (Mayo).... or maybe Sligo if you can find a flight,
(1) Co. Sligo + Strandhill
(2) Enniscrone
(3) Carne's 27
(4) Westport
(5) Connemara + Connemara Isles (9 holes)
depart Galway
for less driving, the all-Donegal trip:
(1) Donegal
(2) Narin & Portnoo + Cruit Island (9 holes)
(3 + 4) Rosapenna, Portsalon
(5) Dunfanaghy or St. Pat's (?)
depart Derry
Or for very little logistical maneuvering:
2-3 days playing Portrush, Portstewart, Castlerock
2-3 days playing Rosapenna, Portsalon
in & out of Derry
There are of course a million options, but those above avoid Kerry and Dublin, since it sounds as if your friends don't want to follow all the tour buses. I suppose they could also do something like Royal Belfast - Bangor - Kirkistown - Ardglass - RCD, which is mostly obscure. (Continental has non-stops from Newark to Belfast, if they go this route, though I understand all the arguments for wanting to avoid EWR.)
Others here can certainly recommend tweaks/substitutions/restaurants, but that should at least get your buddies rushing off to check some websites.
EDIT: The guys who wrote
Links of Heaven (recommended) have a useful list of links up at
http://www.irishgolf.com/related.html