mIRC Portable

https://portableapps.com/search/node/mirc
https://portableapps.com/search/node/irc

http://www.portablefreeware.com/?c=2&sc=75 (down @ time of this posting...)
http://www.csun.edu/~webteach/mirc/mirc57disk.html

From http://ronincyberpunk.com/life/2006/03/03/the-portable-usb-drive/
mirc - As an old time IRC junkie my time on there comes and goes but everywhere I’ve looked 
the irc clients offered aren’t mirc. People don’t realize mIRC is portable, all you need is 
the executable to run it and it will create the .ini files and folders it needs. So I drop 
it on the drive and tote it with me. It’s a bit bloated these days, but it’s what I know and 
I’ll stick with it.

Need I continue?

~Lurk~

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All mIRC does with the registry is store your registration info. All you have to do is copy your mIRC folder and run mirc.exe with -portable in the parameter. After you've done that, your registration is stored encrypted in the mirc.ini file. You may only need to run the mirc.exe with the -portable parameter one time, but I haven't tested it.

So...if you're using PStart for example, you would:
> Copy your mIRC folder to your USB drive.
> Right click the mIRC icon after you've placed it in the PStart menu.
> Select "Properties" and on the "General" tab you just put "-portable" in the "Command Line Parameters" field.