[en] High-spin states in ¹⁷⁸Ir have been produced and studied via the ¹⁵²Sm(³⁰P, 5nγ)¹⁷⁸Ir reaction. In-beam γrays were measured including the excitation functions at 150, 155, 160, 165 and 170 MeV beam energies, K X-γ and γ-γ coincidences at 160 MeV. According to the band structure characteristics and the deduced intra-band B(M1)/B(E2) ratios, two rotational bands are identified for the first time and assigned to be associated with the πh_9/2 direct x νi_13/2 and the πh_11/2 direct x νi_13/2 quasiparticle configurations respectively. Low-spin signature inversion in both bands has been Confirmed based on the observation of signature crossing at I_c = 21 ℎ for the πh_9/2 direct x νi_13/2 semi-decoupled band and at I_c 16 ℎ for the πh_11/2 direct x νi_13/2 strongly coupled band, respectively. Systematics of the signature inversion has been discussed for the semi-decoupled bands in the A = 160-180 mass region and some regularities are proposed concerning the crossing spins with variation of mass. The signature splitting in the πh_11/2 direct xνi_13/2 band of ¹⁷⁸Ir is smaller than that in the similar band of ¹⁷⁶Re; this trend is opposite to the predictions based on the projected shell model