if all interfaces where an vlan is used are using jumbo frames, the irb will have matching MTU.
if interfaces have different mtu (jumbo on one and standard 1500 on another), the irb will inherit the lowest MTU value present on the switch.
Configuring MTU on 'irb' (not irb.<unit>) will not be honored. Trying to set a MTU higher than your interface will even give you a commit warning:
root@labdc01-leaf01# show |compare
[edit interfaces irb]
+ mtu 2500;
root@labdc01-leaf01# commit check
warning: irb.10 ifd's config mtu 2500 is bigger than member link xe-0/0/0.0 ifd's mtu 1514 for family inet
So basically - for irb's you should handle it on the physical interfaces.
et-0/0/48 and xe-0/0/0 have MTU 9216. irb will then have MTU 9206.
et-0/0/48 have MTU 9216 and xe-0/0/0 is default 1514. irb will then have MTU 1500.
Note: Tested on QFX5110 running 17.4R2 but expect exact same behaviour on QFX5200.