Quite a few charity shops won't accept portable electric appliances because they've got nobody to carry out a PAT check on their safety. However that still leaves quite a lot which will accept them (because they've got electricians who volunteer to do the testing).
In mainland UK the British Heart Foundation has specialist shops selling only electrical goods and furniture. They've not got any such shops in Northern Ireland but it might still be worth checking if they'll accept your lamps:
https://www.bhf.org.uk/search/location-search?keyword=belfast&servicetype=books-music-shop,clothing-shop,furniture-electrical-store
The British Red Cross similarly accepts electrical goods in their specialist shops but the only store in Northern Ireland is in Ballymena:
http://www.redcross.org.uk/Get-involved/Our-shops/Our-specialist-shops/Furniture-and-electrical-charity-shops
Oxfam accept electrical items, e.g. at Dublin Road, Belfast:
https://www.oxfamireland.org/shop/oxfam-home-dublin-road
(They'd probably take your lamps at any of their shops, even if that particular shop doesn't sell electrical stuff, and forward them onto an appropriate branch):
https://www.oxfamireland.org/shops
Barnardo's accept electrical goods:
https://www.barnardos.org.uk/shop/donating_goods.htm
Store list here:
http://www.barnardos.org.uk/shop-search-results.jsp?loc=belfast&lg=-5.9301<=54.5973&cl-0=