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Well, it's the last name the county had before ceasing to exist in 1973. Before that, most of it was County Coleraine. Ireland has never had a county called "Derry".

As far as I can make out the names are interchangable depending on your persuasion and the commonly used term both in conversation and on the national airwaves would be Derry.
Not being an expert I'm open to correction, but I knew where the fictious county of Derry is and can even point it out on a map that also calls it Derry <!-- s:? --><img src="{SMILIES_PATH}/icon_confused.gif" alt="" title="Confused" /><!-- s:? -->

Well, its use here is not intended to indicate any persuasion other than the beery sort.

You could follow BBC policy on this issue. The first reference (i.e. the search field) should be Co. Londonderry, all subsequent references should be to Co. Derry.

We all know it was Ryanair who came up with the Londonderry name anyway, Frankfurt (Hahn), Paris (Beauvais), London (Derry).