This is essentially trivial; the only caveat that you need to be aware of is that sometimes mIRC sends the commands faster than the server processes them (I've experienced this on Freenode and one other network). I simply put the necessary commands (the same ones I would type manually) in the PERFORM section of the connection options, each on a /timer - the first command sets a temporary /nick (not on a /timer); the second GHOSTs the "real" nick (timer 5 seconds), the third sets my nick "correctly" (timer 10 seconds), the fourth IDENTIFYs me (timer 15 seconds), and then I have it automatically /join any channels (timer 20 second).
See the Help file for CONNECT OPTIONS, paying special attention to the "reconnect on disconnect" option and the Perform... option. The help for the /timer command might also be useful.
Without giving away my details, what I have in the perform section (and yes, I do it for Freenode) is essentially
/nick TempNickForLogin
/timer1 0 5 /ns ghost RealNick RealPassword
/timer2 0 10 /nick RealNick
/timer3 0 15 /ns identify RealPassword
/timer4 0 20 /join ...
If RealNick isn't there (the ghost has already died, or hasn't been connected in a while), the commands will continue running with no problem; the only issue is if you choose a temporary nick that's used by another user - I tend to just use the first few character of the real nick, followed by the word "login", so if my real /nick is "FudgeFactor" (no, it's not), I'd use (maybe) "FudgeLogin" as the TempNickForLogin in the first line above.
Please be aware that some of the regulars here are likely to be short with you if you haven't made an effort to find the answers to simple questions like this on your own. In such a case - especially if you ask for a script - you may not get more of a response than "/help keyword", where keyword will have useful information for your question, but not necessarily the whole answer.
