I received an email from a parent whose child was competing for the first time in Dunboyne this Sunday as to what their child should wear. This is a great question and my reply to her follows.
Layers are the best thing. If they have an 'under armour' type top or bottoms from other sports or a base layer top from Dunnes, Lidl, aldi, - even a light long sleeved teeshirt or thermal vest something that is tight light and long sleeved, then a short sleeved tee shirt over that then a heavier fleece or tracksuit type top then a light rain/wind proof jacket. Gloves and a hat.
For the legs you can get running type leggings with shorts over or what's easier a light pair of tracksuit bottoms anything that they can run in with freedom. Just before their race they'll take off the jacket and fleece and put on a singlet.
Bring a fresh pair of socks/slippers change of clothes to put on in the car going home just in case they get their feet wet. Towels too might be needed. :)
As a treat have some hot chocolate in a flask with some cookies or scones sandwiches to have in the car after (if very organised they can make them in the morning :) :) ) so that the experience becomes a day out with the family.
For yourself wrap up very warm gloves hat rain leggings big umbrella etc (you could bring a flask with hot coffee nothing nicer) as you'll not be running like them.
It's all about being prepared for what the Irish weather throws at us. We were very fortunate last year that the weather was good for all the competitions but nothing is guaranteed.
I hope this helps,