  The aim of this paper is to study the environment and intervening absorbers
of the gamma-ray burst GRB 090926A through analysis of optical spectra of its
afterglow. We analyze medium resolution spectroscopic observations (R=10 000,
corresponding to 30 km/s, S/N=15 - 30 and wavelength range 3000-25000) of the
optical afterglow of GRB 090926A, taken with X-shooter at the VLT ~ 22 hr after
the GRB trigger. The spectrum shows that the ISM in the GRB host galaxy at z =
2.1071 is rich in absorption features, with two components contributing to the
line profiles. In addition to the ground state lines, we detect C II, O I, Si
II, Fe II and Ni II excited absorption features. No host galaxy emission lines,
molecular absorption features nor diffuse interstellar bands are detected in
the spectrum. The Hydrogen column density associated to GRB 090926A is log
N_H/cm^{-2} = 21.60 +/- 0.07, and the metallicity of the host galaxy is in the
range [X/H] =3.2X10^{-3}-1.2X10^{-2} with respect to the solar values, i.e.,
among the lowest values ever observed for a GRB host galaxy. A comparison with
galactic chemical evolution models has suggested that the host of GRB090926A is
likely to be a dwarf irregular galaxy. We put an upper limit to the Hydrogen
molecular fraction of the host galaxy ISM, which is f < 7X10^{-7}. We derive
information on the distance between the host absorbing gas and the site of the
GRB explosion. The distance of component I is found to be 2.40 +/- 0.15 kpc,
while component II is located far away from the GRB, possibly at ~ 5 kpc. These
values are compatible with that found for other GRBs.
