The underage categories are explained  below:-
Junior:

If you turn 17 or 18 years in the calendar year that you are racing. You can either get a non-competition licence (€25 – or €15 if you are a new Cycling Ireland member) or a full competition licence (€50). As a junior you will race with the seniors, and the grades vary from A4 (entry level junior men, and most junior women), to A3 (the majority of Junior men, and the top junior women) and A2 (the top junior men). First year junior women (as in girls who turn 17 in the calendar year in which they race) can race with U16 men.

Youth:

U16

You are U16 if you turn 15 or 16 in the calendar year in which you are racing. Youth riders do not race with the seniors. As a youth you can either get a non-competition licence (€25 or €15 if you are a new member with Cycling Ireland) or a full competition licence (€35). Girls who are U16 can race with U14 boys.

U14

You are U14 if you turn 13 or 14 in the calendar year in which you are racing. U14 licences cost €5 and are full competition licences. U14 girls can race with U12 boys.

U12

You are U12 if you turn 11 or 12 in the calendar year in which you are racing. U12 licences cost €5 and are full competition licences.

U10

Anyone under the age of 10 can take out a U10 licence, but note that you cannot race in road races until you pass your tenth birthday.