You are absolutely right, of course Derry wasn’t part of the destroyer-for-bases deal.
By 1941 Derry was the biggest convoy escort bases for the Royal Navy and Royal Canadian Navy. With the loss of the Irish Treaty Ports Derry became the furthest west port available to the British on the Western Approaches but it was an inconvenient little port badly in need of upgrade.
I don’t think it can be held against FDR if he did some forward planning to secure the North Atlantic convoy routes across which it was fairly obvious millions of American boys would soon be transported.
Oh, I’m not faulting FDR. I think he was simply being a realist about America’s prospects for staying out of the war.
It would have been a disaster for the free west if England had been conquered by Germany. It was bad enough to see France go.
Anyone familiar with Halford Mackinder’s geographical pivot of history could read the global map. Virtually all of the military knew what it is and I’m sure FDR was familiar with it as well.