I know you can lock mIRC while you're in it but is it possible to make a password before you can open mIRC?

Try this:

alias password {
if (%password == on) { goto login }
if (%password == off) { halt }
:login
.showmirc -t
/set %times 1
/set %passwordtry1 $?*="Password:"
if (%passwordtry1 == %yourpass) { goto success }
if (%passwordtry1 != $1) { goto login2 }
:login2
/set %times 2
/set %passswordtry2 $?*="Password:"
if (%passwordtry2 == %yourpass) { goto success }
if (%passwordtry2 != $1) { goto login3 }
:login3
/set %times 3
/set %passwordtry3 $?*="Password:"
if (%passwordtry3 == %yourpass) { goto success }
if (%passwordtry3 != $1) { goto sorry }
:sorry
if (%times == 3) { /exit }
:success
.showmirc -s
/unset %passwordtry1
/unset %passwordtry2
/unset %passwordtry3
/unset %times
}

menu status,menubar {
Password System ( $+ %password $+ )
.On:/set %password on | /techo Password is now ON
.-
.Off:/set %password off | /techo Password is now OFF
.-
.Change Password:/newpass
}

alias techo {
if (%scriptstamp == 1) { echo -at %s $1- }
else { echo -a %s $1- }
}

alias newpass {
/set %newpasstry $?*="Old password:"
if (%newpasstry == %yourpass) { goto change }
if (%newpasstry != $1) { goto sorry }
:change
/set %yourpass $?*="New password:" | halt
/unset %newpasstry
:sorry
/techo Wrong password
/unset %newpasstry
}

on 1:START { /password }

Very messy all you need is
alias password {
if (%password != on) return
.showmirc -t
var %times = 1
while (%times <= 3) {
if ($?*="Password:" == %yourpass) { .showmirc -s | return }
inc %times 1
}
exit
}

i like the first one better.

There really ought to be a way to /showmirc -n or -t (minimized or tray) and specify that it goes into Lock Mode as if you were holding the CTRL key.

/showmirc -nrstxoplL
Manipulates the display of the main mIRC window, where -n = minimize, -r = restore, -s = show, -t = tray, -x = maximize, -o = on top, -p = not on top, -l = minimize & lock, -L = tray & lock.

- Raccoon

This suggestion posted on the Feature Suggestions forum.

you could just go to options > general > lock, set the password, and check the box next to "ask for password on startup"

Yes, and if I want to issue this by /command? Say after I idle for 60 minutes, I want mIRC to auto-lock?