We report the results of quantum chemical calculations, variously using the B3-LYP, MP3, CCSD and CCSD(T) levels of theory in conjunction with double-zeta, triple-zeta and augmented basis sets, on the structures and energies of MgC3N isomers. The global minimum on the [MgC3N] potential energy surface is linear MgCCCN (2Σ), with all other feasible isomers lying at least 80 kJ mol-1 higher in energy. When the optimized geometries obtained are adjusted appropriately (as judged according to the scaling factors required to rectify theoretical rotational constants for the species MgCN, MgNC and MgCCH), the levels of theory deliver values for the rotational constant of MgCCCN (2Σ) ranging from 1.383 to 1.394 GHz. All such values lie within +/-0.8 per cent of the rotational constant of an unidentified linear (2Σ) species, B1395, seen in the envelope of IRC+10216, for which we provide observational details. Other candidates for B1395 include an excited-state species C6H (2Σ+) and, as we note here, MgCCCCH (2Σ) nevertheless, MgCCCN - which has already been predicted as a possible constituent of IRC+10216 - is the most promising candidate for a radioastronomical detection in this object by virtue of its high dipole moment and the putative formation mechanism which it shares with the previously detected species MgCN and MgNC.