Route where you can, bridge where you must.
You could enable routing on the devices and use a /30 for the serial link, adding static routes or whatever dynamic routing protocol you have on the networks at both ends.
Another possibility would be to use "ip unnumbered ethernet [whatever]" on the serial link and route as appropriate to the interface.
Either of these will give you an IP address on each box for management and monitoring without having to use IRB.
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