The properties of two-dimensional supramolecular self-assemblies on surfaces depend on the fine balance between molecule substrate and molecule molecule interactions. In this article, we study the growth of 1,3,5-tri(4'bromophenyl)benzene (TBB) monolayer on graphene epitaxially grown on Ir(111) by means of low temperature scanning tunneling microscopy and spectroscopy (LT-STM/STS) in interaction with the Gr/Ir(111) substrate, using density combined with a fully atomistic description of the molecules functional theory (DFT). In order to figure out the impact of the underlying metallic layer upon the self-assembling behavior of the molecules and their properties, we compare our results with those theoretically obtained on pristine graphene or experimentally achieved on highly oriented pyrolytic graphite (HOPG). We demonstrated that the use of the Ir layer allows the formation of large extended, continuous, and two-dimensional supramolecular networks lying even over Ir step edges like a carpet. In addition, we highlighted the obtention of two structural polymorphs never observed' on HOPG. In the light of DFT simulations, we assumed that the formation of these polymorphs is driven by the balance between molecule molecule interactions, due to halogen bonds (XB), and the tailored molecule surface interactions due to the presence of Ir layer.