I don't think RCD or Portrush would get a look in with the present hotel provision and transport infrastructure. I don't think RCD would want it - certainly in the days when I played it a bit the reason given was that they didn't want damage to the flora and fauna.
Infrastructure is the reason only one course in the south of England is currently on the roster. Perhaps Deal will be brought on board. Turnberry was omitted for some years for the same reason
Troon seems to get a low rating architecturally, but shots such as the approach to the 10th green or finding the 13th fairway from a back tee can be as exciting as they come. The front nine is fairly mundane, with a particularly notable exception. I loathe the 9th and have never understood how anyone can play the hole in par, but they do. The two par 5s going out are pretty dull, but I love the short 5th between them and the 7th is one of golf's loveliest holes. The first 3 are a walk-over for the big boys, but allow people like me to be broken in gently. That, of course, leaves the Postage Stamp, a quite brilliant hole. I know I've said this before, but it's worth playing Troon in the winter in order to play the original hole, which is beyond and to the left of the big mound normally on the left of the green. It's a wicked hole!
Birkdale is the other course which doesn't set many pulses racing on GCA. I have loved every round I've played there and have always found the course to have many more subtleties than might appear at first glance. I think Martin Hawtree did an excellent job of rebuilding the greens. I've played it quite a few times in mixed golf, and then you notice further subtleties. Take the 14th, for instance, a mid-length par 3 across a valley to a tipped-up green. A competent lady golfer, but not a particularly long hitter, unable to clear the green-front bunkers is forced to hit a wood towards the left front of the green, run it through the gap and then watch as the ball climbs the slope of the green, veering all the time to the right, until it trickles down to the hole having turned through 180-degrees in the course of its journey. I was at Birkdale when O'Meara won the Open and it was interesting to watch players then at the peak of their career struggling around the greens - few could get the ball as close to the pin as we might have expected them to. OK, there were some fabulous rescue shots - Rose from the 18th rough and (especially) Watts from a greenside bunker on the 18th - but the general success rate suggested to me that the greens were more mischievous than some give them credit for. And for spectators there are few better courses for visibility.
Now, those who complain that Birkdale's fairways are just too flat and fair and don't have the unpredictability of true links golf, are Birkdale's fairways any flatter than Hoylake's?