Ferromagnetic shape memory alloy of nominal composition Ni50Mn35Sn15 has been investigated by magnetic and transport measurements. Clear signature of first order martensitic transition is observed over a region around 180 K in resistivity, dc magnetization and ac susceptibility data. Field-cooled and zero-field-cooled magnetizations diverge below the martensitic transition, signifying magneto-thermal irreversibility originating from pinning by variants. The sample shows large negative magneto-resistance (-25% at 90 kOe) in the region of martensitic transition, which was found to be highly irreversible. A clear correspondence between magnetoresistance and dc magnetization is observed above the field of technical saturation.