I might be wrong, but i seem to remember during the certification process of the 757-300, Boeing deliberately chose Cork in the winter to test the ability of the aircraft in crosswinds/foggy conditions for a few days. I suppose that for the BBJ the choosing of Cork was as mentioned, ease of use and that it being so southerly, there would be few restrictions straight in off the Atlantic and then out sidestepping most of the southern UK airspace and over to Paris.
SHJ