This article explains what is required to bring an IRB interface unit up for Juniper Networks EX/QFX Series Switches. 

The IRBs link state is down after being configured because the link status of the interface which should allow the vlan is down, or the IRB interface is not associated to a VLAN.

{master:0}

root@switch> show interfaces irb terse

Interface Admin Link Proto Local Remote

irb up up

irb.41 up down inet 10.10.10.1/24

To configure the routed VLAN interface:

• Create the VLAN by assigning it a name and a VLAN ID:

[edit]

user@switch# set vlans support vlan-id 111           

• Assign an interface to the VLAN by specifying the logical interface (with the unit statement) and specifying the VLAN name as the member:

[edit]

user@switch# set interfaces ge-0/0/18 unit 0 family ethernet-switching vlan members support           

• Create the subnet for the VLAN’s broadcast domain:

[edit]

user@switch# set interfaces irb unit 111 family inet address 10.10.10.1/24           

• Bind a Layer 3 interface with the VLAN:

[edit]

user@switch# set vlans support l3-interface irb.111

NOTE: At least one port (access or trunk) with an appropriate VLAN assigned to it must be up for the irb interface to be up. That is, if you assigned irb.111 to VLAN 111, you must confirm that the interface that allows VLAN 111 is up. Otherwise, the IRB interface will remain down despite everything being configured correctly.

https://www.juniper.net/documentation/us/en/software/junos/multicast-l2/topics/topic-map/irb-and-bridging.html