  We consider the non-equilibrium dynamics arising after a quench of the
transverse magnetic field of a quantum Ising chain, together with the sudden
switch-on of a long-range interaction term. The dynamics after the quantum
quench is mapped onto a fully-connected model of hard-core bosons, after a
suitable combination of a Holstein-Primakoff transformation and of a
low-density expansion in the quasi-particles injected by the quench. This
mapping holds for a broad class of initial states and for quenches which do not
cross the critical point of the transverse field Ising model. We then study the
algebraic relaxation in time of a number of observables towards a metastable,
pre-thermal state, which becomes the asymptotic steady state in the
thermodynamic limit.
