Integrated routing and bridging (IRB) is a feature of Cisco Firepower Threat Defense (FTD) that allows the firewall to forward traffic at both layers 2 and 3 for the same subnet. In this mode, the firewall can act as a switch or a bridge to forward traffic at layer 2 and as a router to forward traffic at layer 3. This allows the firewall to maintain full control over the traffic, while still allowing it to forward traffic at both layers.
https://www.cisco.com/c/en/us/td/docs/security/firepower/ftd-config-guide/FTD-Config-Guide-v6/Integrated-Routing-and-Bridging.html