1) Go to the very top left hand corner of mIRC and left-click on the mIRC icon there. Then check the menu item 'On Top'.
2) You cannot set transparency for the main mIRC window anymore, see
this thread.
Transparency for
desktop windows can be set in
ALT+O >
Display >
Options >
[Windows...] button, then set the transparency per centage (it's the option at the bottom).
Regards,