It can be difficult to choose books as a gift unless you know exactly what the child is currently into. Maybe go the route of a book token? It's possible to buy ones that can be used in pretty much every book shop in the country. Spend maybe €10 on the token, then wrap it with a bar of chocolate or a selection box (€2 in Euro€2 and other assorted discount shops) to bulk it up a little

(I've seen kids of assorted ages getting Easons giftcards taped to a big Toblerone bar, and they were all delighted).
Debenhams have a big range of gadgets in the basement of the Henry Street store. Most are above your budget, but there is a savings jar that counts the money you put into it, and allows you to adjust the balance if you take anything out. From memory, that was about €12 (currently 25% off). They also sell a big range of Top Trumps cards, that might go down well (I'm a grown-up, and still get a kick out of playing these)
For nieces, try looking in Boots for gift sets of make-up or glittery bubblebath - girls can never have too much of this stuff! You could also check out the Totes range of slipper socks - also available in Boots. Practical yet fun

And most gifts in Boots are 3 for 2.
Hats, scarves and gloves are always useful, and again, particularly for girls, always welcome. Pennys do some lovely ones that aren't overly expensive.
Hope some of these suggestions help.