When starting out, for day hikes, we suggest using existing backpack style schoolbags before rushing to buy extra gear.
Definitely a proper pair of boots that fit and decent comfortable socks (and a spare pair to change if necessary).
Waterproofs - a jacket and trousers. Mac-in-a-sac style are ok for starting, especially trousers - but if they stick with it, may need better quality in time. Aim for ones that are breathable so they don't just sweat underneath, and get as wet as if in the rain (although not as cold because the jacket keeps the wind off - but they will get cold very quickly when they stop walking!).
Layers - a top that wicks sweat away is great. A jumper/hoodie etc over that. A spare extra layer in the bag if necessary - perhaps a fleece etc. Perhaps a thermal layer on colder days, but you heat up a lot when walking.
Walking trousers, or tracksuit bottoms. NOT Jeans!
Good materials are merino, synthetic active wear fabrics etc, sports wear fabrics, but cotton holds moisture so isn't great.
A wooly hat - just a beanie hat or similar - and gloves
Water bottle - preferably 1 litre sized.
Snacks
Torch and personal first aid kit (just in case)
Pocket pack of tissues (useful for purposes other than just wiping noses....)
Lip balm, sun cream etc
As you are in Ireland,
Decathlon is in Belfast, and opening in Dublin soon =- we've bought loads of their gear for outdoor use (Scouting and sailing) and it's generally really good.
Have you checked the Scout Shop itself?
Lidl camping gear is quite decent - they usually have a few weeks of it across spring/early summer - lots of my Cubs parents have got sleeping bags and sleeping mats there. And we've got great waterproofs over time for DD there too.
And Aldi is worth considering too. I don't know about their clothing, but I have got a few other bits there which were fine (eg. mini bbq which comes on all my non-Scout camp trips).
Great Outdoors (Cork and Dublin at least) and Cotswold (certainly has shops in Andrews St and Dundrum SC in Dublin, and also online - great "sale" section online!) both have 10% off for Scouts (on full price items) - bring the Scouts' necker with you just in case they ask for "proof", but can be useful.
Mountain Warehouse is also good for cheaper gear, I've got lots online and they also have a store in Blanchardstown, and possibly a few others around Ireland.
Regatta and Trespass both have more outlets around the country, and their gear tends to be a bit cheaper also.