  We report the measurement of the non-reactor environmental backgrounds and
the detector response with the Indian Scintillator Matrix for Reactor
Anti-Neutrinos (ISMRAN), which is $\sim$1 ton detector setup by volume,
consisting of 10$\times$9 (10 rows and 9 columns) Plastic Scintillator Bars
(PSBs) array at BARC, Mumbai, India. ISMRAN is an above-ground anti-neutrino
($\mathrm{\overline\nu_{e}}$) experiment at very short baseline located at
Dhruva research reactor facility. It is enclosed by a shielding made of 10 cm
thick lead and 10 cm thick borated polyethylene to minimize the backgrounds and
is mounted on a movable base structure, situated at $\sim$ 13 m away from the
reactor core. These measurements are useful in the context of the ISMRAN
detector setup that will be used to detect the reactor
$\mathrm{\overline\nu_{e}}$ and measure its energy spectrum through the inverse
beta decay (IBD) process. In this paper, we present the energy resolution model
and energy non-linearity model of PSB and the cosmogenic muon-induced
background, based on the sum of their energy depositions and number of hit
bars. Reconstructed sum energy spectrum and number of hit bars distribution for
$\mathrm{{}^{22}Na}$ radioactive source has been compared with Geant4 based
Monte Carlo simulations. These experimentally measured results will be useful
for discriminating the correlated and uncorrelated background events from the
true IBD events in reactor ON and OFF conditions inside the reactor hall.
