Intelligent Reflecting Surface (IRS) is one of the promising technologies for improving transmission reliability of Industrial Wireless Sensor Networks (IWSNs) by creating a virtual line-of-sight (LoS) link to bypass obstacles between transceivers. In this paper, we consider an IRS-assisted IWSN and propose a resource allocation method for reliable transmission in the IRS-assisted IWSN. First, a transmission reliability optimization problem in terms of the reflective beamforming at the IRS and the timeslot and power allocated to sensors is formulated. Second, the formulated optimization problem is transformed into an equivalent but simplified problem. In subsequent, a particle swarm optimization based method is utilized to obtain a near-optimal solution. Simulation results demonstrate that this work yields higher transmission reliabilities than existing benchmark schemes.