Version: 2.8.128

From Client:
Typing /servirc alias <new-custom-nick> returns That player cannot be found.

Typing /servirc alias <official-mc-name> <new-custom-nick> returns:

Alias for <old-custom-nick> set to <new-custom-nick>
<old-custom-nick> is now known as <official-mc-name>

so whatever I do, my name is always the official-mc-name

Tested just now on 2.8.154 and it worked. Not sure if I maybe fixed it somehow already.
If it still won't work for you on that version, it could be that some other mod is messing with your name at a higher priority order (unlikely though since EiraIRC runs as highest for name changes).