The bases you see listed in the Destroyers For Bases 1940 agreement are in Canada and the Caribbean, but I’d bet that your dad saw something that was off the books. FDR was trying to help the UK all that he could under the limitations that Congress had on our involvement.
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.