  The minimal effective model for the magnetisation plateaus below 1/2 in the
Shastry-Sutherland model is derived to be an Ising model of certain unit-cell
hard-core bosons with anisotropic repulsive interactions on isosceles
triangular lattice. It unambiguously gives the prominent plateaus at 1/8, 1/6,
1/4, 1/3 and an additional one at 3/8, related through a particle-hole (p-h)
transformation. The Dzyaloshinsky-Moriya (DM) interaction dresses it up with an
inhomogeneous transverse field that also causes p-h asymmetry due to an
in-plane component of the inter-dimer DM vector. This explains the asymmetry
between the magnetisation below and above 1/4 in SrCu$_2$(BO$_3$)$_2$. The
effective model above 1/2 plateau is an XXZ model with stronger XY parts. It
gives no magnetisation plateaus, but exhibits chiral order.
