Hi there,
I just bought a Pironman 5 plus power adapter and assembled it. Now when I power it on, it boots into Raspbian but as soon as I connect a keyboard and/or mouse it almost immediately goes into “undervoltage detected”. How can that be? Because I bought the Pironman power supply, too …
Besides that, I have no clear pointers that either a mouse nor a keyboard work … Any help/advise?
Is there wired or wireless keyboard\mouse? By my side, I use wireless and I didn’t get any issue. Do you have original pi5 power supply 27w PD?
You can even take a look into raspberry configuration \performance to unable usb current limitation.
Or if you can access to your pi5 by SSH or VNC, try to update all system and kernel
sudo apt-get update && sudo apt full-upgrade
sudo rpi-update and reboot.
I am using the one, which you can buy along in the Pironman bundle … It says “Combo: Pironman+5.1V5A 27W Power Adapter (EU)” … hence I would have expected to see no issue at all in that direction 
Good news, you have the appropriate power supply. Now the question is, are you using wired or wireless keyboard \mouse?
For my first test I connected my apple keyboard via usb to the pironman … For the mouse, I would have a wireless one … However, I am having hard times to let it look for a bluetooth device when I cannot navigate at least with my keyboard 
You can connect with vnc, idk if there apple apps VNC, with it you can control pi with your phone.