IRB main usage is that the decision whether to route or to bridge is no longer necessarily a global decision (by either enabling routing or disabling it) but can be made interface by interface so that some interfaces can route while other interfaces bridge the traffic.
Your network might require you to bridge local traffic within several segments and have hosts on the bridged segments reach the hosts on routed networks and here the job of IRB comes, using IRB, you can route a given protocol between routed interfaces and bridge groups.
 To add more, because bridging is in the data link layer and routing is in the network layer, they have different protocol configuration models. With IP, for example, bridge group interfaces belong to the same network and have a collective IP network address. In contrast, each routed interface represents a distinct network and has its own IP network address. IRB uses the concept of a Bridge Group Virtual Interface (BVI) to enable these interfaces to exchange packets for a given protocol. A BVI is a virtual interface that acts like a normal routed interface. A BVI does not support bridging but actually represents the corresponding bridge group to routed interfaces.
As Mohammed pointed out, IRB is designed in Access Points to provide scalable configurations.
For instance, you want your wireless network to be the same (IP,IPX,Appletalk,etc) as your wired network. On this configuration, the BVI interface which represents the IRB design is simply bridging between the two interfaces (radio antenna and AP LAN interface).
If you want the wireless network to be in a different network, the BVI interface will route packets between the two interfaces (radio antenna and AP LAN interface).
Keep in mind, if you set your AP in routed mode, certain protocols may be unsupported such as IPX and Appletalk among others.
We are pleased to announce availability of Beta software for 16.6.3.
16.6.3 will be the second rebuild on the 16.6 release train targeted
towards Catalyst 9500/9400/9300/3850/3650 switching platforms. We are
looking for early feedback from customers befor...
Introduction Featured Speakers Luis Espejel is the Telecommunications
Manager of IENova, an Oil & Gas company. Currently he works with Cisco
IOS® and Cisco IOS XE platforms, and NX to some extent. He has also
worked as a Senior Engineer with the Routing P...
In this session you can learn more about Layer 3 multicast and the best
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