I tried a number of places in the border area when recently buying a chest freezer, mielle vaccuum and tumble drier and i found Dalzells of Markethill to be the cheapest (i was able to buy high spec Whirlpool A+ rated chest freezer considerably cheaper than the basic powerpoint type models in the ROI). The are good to deal with and can be bargained down from their list prices.
Currys also have very good special deals but for general stuff can actually be quite expensive compared to the Northern Irish independant stores.
Watters are also a worth a call to though.
For TVs and brown goods generally Play.com (stg) and Amazon are hard to beat.