You can play almost all the Irish courses as a single (RCD, Portrush, Lahinch included). Portmarnock and Ballybunion can be hard to access even as a single but if you are willing to wait that is also often do-able. The Scots and Irish have become drugged by the American and Japanese dollar and Yen. Be prepared to pay $100-200+ for a round. $300 at Old Head.
Have a letter of intro with handicap and most will let you fill out a foursome. In Ireland and Scotland you generally have to be "invited" to join a 3-some, in other words the group is asked and can refuse you if they wish.
JC