You can't register for rides. You can document the ride per the IBA's guidelines and submit that in one of a few different ways after you do the ride.
Start your reading
HERE. The Electronic method is
HERE.
You're probably aware, Ireland is an island... You will need to use care to document your Dated Business Receipt stops to demonstrate you rode the path you chose to ride. Repetitive circular routes are frowned upon, so try to use as much of the length of the island as you can, within reason for where fuel is available and the time line involved. Looks like it will be four legs if you ride the length of the island and back twice.
MAP EXAMPLE
Please ask more questions if you have them, and feel free to share a link to the route you wish to ride via Google Maps. Remember to add a 3% to 5% buffer on your mileage to ensure you do exceed 1000 miles.
Also of importance, Ireland uses Km instead of Miles, as I understand. Please correct me if I'm mistaken. If that is the case, you would want to submit a SS1600 kms ride, not a SS1000 ride. Link
HERE. The KM series of rides is specifically for rides outside the USA where Kms are the normal standard for measuring distance.