I recently acquired a SCSI flatbed scanner for my computer. The computer already has a SCSI DAT tape drive attached, so do I need to modify the IRQ numbers for each device (e.g. IRQ 1 for the scanner, IRQ 2 for the tape drive)?

I think you are a little confused between IRQ numbers and SCSI identification numbers. When using the SCSI interface, each device is assigned a particular SCSI identification number (or ID). Each device is assigned a particular ID number based on their position in the SCSI chain. This ID number must be unique to the device, otherwise the device will not be recognised (nor will the other device which is sharing that same ID number). The ID number is generally set using jumpers on the SCSI device, so you will need to refer to the manual for your scanner and tape drive to determine how to set the ID number. However before doing so, you should read the manual for your SCSI controller card (adapter) in the computer, as the exact method to configure the SCSI IDs is dependent on the controller card. For example, serial SCSI buses can automatically assign the IDs to each device, whereas parallel SCSI buses require the ID to be manually set for each device. Also, generally the SCSI controller needs to be assigned a particular ID, which varies between different controllers. So, you will need to consult the manual for your controller card to check for these things. You should also be aware you may need to terminate the SCSI chain after the final device, should the final device in the chain not have an internal termination capability. Termination is designed to stop signals from ‘echoing’ (or ‘reflecting’) over the SCSI bus. Terminators are generally placed at two positions on the bus - the beginning and the end. The SCSI host controller itself (which the SCSI chain is plugged into) normally functions as the terminator at the beginning of the chain, and you then need a terminator at the end of the chain (which may be either a device which has internal in-built termination, or an external terminator you plug into the end of the chain).