The next step up from the basic Circuit of Ireland is a Saddlesore 1000 ( 1000 miles in less than 24hrs) or a SS1600k (1610km in less than 24hrs). Again, completion of this ride will gain you membership of the IBA if you are not already a member.This is a bit more difficult to achieve within Ireland.You will be looking at around 20+ hours riding.There really isn't much of a choice within Ireland when it comes to selecting a possible route for a Saddlesore ride .The Iron butt Association isn't keen on routes that are overly repetitive in nature so 10 laps of a 100 mile circuit is not acceptable.
The important thing is to make sure the route you pick is well over the 1000 mile or 1610km mark. Nothing worse than riding a route which actually measures 987 miles. Mapping software is used to verify mileages so never ever rely on your odometer, which is usually overly optimistic. Also remember that you need to verify any "corners" in your route. Unless you prove your route by receipt, the verification team will just calculate the shortest possible route between the points.
We would expect that rides starting in the south of Ireland will be conducted in KM whilst those originating in the North would be conducted in Miles. But it's up to you how you want to measure your ride. The only difference will be the certificate.