We report a strong correlation between the location of Mn sites in ferromagnetic $Ga1−xMnxAs$ measured by combined channeling Rutherford backscattering and by particle-induced x-ray emission experiments and its Curie temperature. The concentrations of free holes determined by electrochemical capacitance-voltage profiling and of uncompensated $Mn2+$ spins determined from superconducting quantum-interference device magnetization measurements are found to depend on the concentration of unstable defects involving highly mobile Mn interstitials. This leads to large variations in $TC$ of $Ga1−xMnxAs$ when it is annealed at different temperatures in a narrow temperature range. The fact that annealing under various conditions has failed to produce Curie temperatures above ∼110 K is attributed to the existence of an upper limit on the free hole concentration in low-temperature-grown $Ga1−xMnxAs.$