Some great replies already people - keep it up!!
Car Boot Sales:
Antrim (Dunsilly, roundabout):
My absolute favourite CBS, in the summer the field behind the car-park is jam packed with sellers. Get there for 6 on a Saturday (or before!!) if you want the bargains because this one is a buyers market.
Ballymena (near cinema):
Saturday AM. More of a sellers market than buyers, with some regular collectables / antiques traders in attendance. Check out Nicky Brady's lot as he is easy to deal with. You do get non-traders as well and this one is packed in the summer.
Benvardin Dogs Home, Ballybogy. Sunday AM. Great all-round CBS (best from April to Oct.) where you can get decent priced collectables on one stall, Chickens on the next!!, and CDs etc. on the next. A real mixed bag – £1 entry fee (per car) goes to the Dogs Home.
Greysteel, Co. Derry. Sunday AM. For me this is the best 'sellers market'. 50p per adult entry fee (£5 traders per car, vans and trailers + another £5). Packed in the summer months and always a good run for those who don't live near the border areas, as Muff is only 10mins away for diesel / petrol.
Newtownards (Portaferry Rd.). Sunday AM. Like the others good from April to Oct. and can be combined with the Windsor Park CBS as well.
Others worth a mention, Windsor Park Sunday AM, Nutts Corner Sunday PM (more of a traders market than CBS), Toome, Elk Car-park (1st Saturday AM of every month), Ballymoney Church of God (1st Saturday AM of every month – I think!), Moneysallin (near Kilrea) Saturday AM. Lifford (Sun AM) – need to check details on this one. Doccyboy you mentioned Newcastle – is this on a Saturday?
Please note I will update this list through time and if I missed some boot fairs – please post details and I will update.
ps - the links are to multimap locations (apologies for pop-ups)
pps- check out Orange Hall auctions that occur about once or twice a year - if you want absolute bargains then these auctions is where you'll find them (especially collectables)