  Isotopologue abundance ratios are important to understand the evolution of
astrophysical objects and ultimately the origins of a planetary system like our
own. Being nitrogen a fundamental ingredient of pre-biotic material,
understanding its chemistry and inheritance is of fundamental importance to
understand the formation of the building blocks of life. We present here
single-dish observations of the ground state rotational transitions of the
$^{13}$C and $^{15}$N isotopologues of HCN, HNC and CN with the IRAM 30m
telescope. We analyse their column densities and compute the $^{14}$N/$^{15}$N
ratio map for HCN. The $^{15}$N-fractionation of CN and HNC is computed towards
different offsets across L1544. The $^{15}$N-fractionation map of HCN shows a
clear decrease of the $^{14}$N/$^{15}$N ratio towards the southern edge of
L1544, where carbon chain molecules present a peak, strongly suggesting that
isotope-selective photodissociation has a strong effect on the fractionation of
nitrogen across pre-stellar cores. The $^{14}$N/$^{15}$N ratio in CN measured
towards four positions across the core also shows a decrease towards the
South-East of the core, while HNC shows opposite behaviour. The uneven
illumination of the pre-stellar core L1544 provides clear evidence that
$^{15}$N-fractionation of HCN and CN is enhanced toward the region more exposed
to the interstellar radiation field. Isotope-selective photodissociation of
N$_2$ is then a crucial process to understand $^{15}$N fractionation, as
already found in protoplanetary disks. Therefore, the $^{15}$N-fractionation in
pre-stellar material is expected to change depending on the environment within
which pre-stellar cores are embedded. The $^{12}$CN/$^{13}$CN ratio also varies
across the core, but its variation does not affect our conclusions on the
effect of the environment on the fractionation of nitrogen.
