Probably best to post your application in. the computerized version is a farce. If it's their first time, you will also need a copy of a birth cert or passport to prove their age.
the application forms link is somewhere here on this forum.....
I suggest you contact Cycling Ireland.
For Under 18 just apply for the full licence, the offroad restricted licence just applies to adults, as the cost for the overall one of under 18s is lower than the offroad one.
When you complete this form, you are emailed instructions on what to do next, a copy of the form also gets emailed to Cycling Ireland to let them know of your application.
Hopefully this will help streamline the application process for offroaders, as the process on the CI site is just too confusing for many applicants (including myself!)
