  We examine ADHM multi-instantons in the conformal N=2 supersymmetric Sp(N)
gauge theory with one anti-symmetric tensor and four fundamental
hypermultiplets. We argue that the ADHM construction and measure can also be
deduced from purely field theoretic considerations and also from the dynamics
of D-instantons in the presence of D3-branes, D7-branes and an orientifold
O7-plane. The measure then admits a large-N saddle-point approximation where
the D3-branes disappear but the background is changed to AdS_5 x S^5/Z_2, as
expected on the basis of the AdS/CFT correspondence. The large-N measure
displays the fractionation of D-instantons at the singularity S^3 in S^5/Z_2
and is described for instanton number k by a certain O(k) matrix model.
