The answer is probably simple, and one word. Encodings.
On your mirc client, did you enable utf-8 support? When setting the password are you using a language set other than english? Extended Symbols perhaps? Weird Characters?
You must apply these same settings to irsii if you answer yes. If you cannot apply these settings, you will need to type your password into an encoding convertor (find these online), and convert from the mirc encoding which was used (utf-8?) into the encoding irsii uses (unicode?). Then use that transcoded password to give the bot. Afterwards, change the password once your on the partyline:
.chpass <transcoded-old-pass-same-one-which-got-you-on-the-partyline> <new-pass-of-your-choosing>
also, see
here for more info about the problems with irsii and encodings.