Support for LIRC has been removed in mpv 0.9.0. The new way to use such devices is through evdev. This means the devices will emit normal X11 key events. You should be seeing these key press events if you start xev (make sure xev is focused). This might require additional configuration with ir-keytable.
Likewise, if you use them in mpv, mpv should complain about an unmapped key. You can use the printed key name or code and add a binding to input.conf. You can also see all key events mpv receives with mpv --input-test --force-window --idle.
Some IR devices (in particular very old ones) might not work.
If you're using lirc, irxevent can be useful in routing appropriate commands to mpv:
begin
prog = irxevent
button = Play
config = Key space mpv
end
begin
prog = irxevent
button = Volume+
config = Key 0 mpv
repeat = 1
delay = 1
end
Alternatively, and since mpv cannot natively bind global key bindings, if you really need to control mpv while mpv is not focused, consider writing a script that controls mpv through a FIFO or JSON IPC.