County Londonderry Coat of Arms

I have no information on the origin or meaning of these arms - can anyone please help?

Each IBD County Page has or will have (where possible) the relevant Coat of Arms. This is made possible thanks to a fantastic heraldry website called the International Civic Heraldry Website. Sometimes a description is not given due to lack of information, so if anyone could help it would be greatly appreciated. You may visit this superb site by using the link below.