All we know about this Lady is that she "donated" the Elfin Oak (this probably means that she funded its removal from Richmond Park and/or its carving by Innes) and that she was present at the unveiling on 25 July 1930.
At one time we were convinced that this Lady was Emily Ormsby-Gore (1856 - 1929) (married Hugh Fortescue, 4th Earl Fortescue in 1886) and also that she was the donor of the Time Flies clock tower. Do read her page and assess our argument. However Lady Fortescue was reported as being present at the unveiling, a year after Lady Emily had died. So we have to discount her.
Other candidates: Margaret Beaumont (1892 - 1958) (married Hugh Fortescue, 5th Earl Fortescue in 1917); Winifred Beech (? - 1951) (married Sir John William Fortescue in 1914); all the unmarried daughters of the Earls were also entitled 'Lady Fortescue'; and no doubt there are others we have not found.