To check whether the "on top" setting is working correctly...
If you turn "on top" on and then try to switch to other applications, does mIRC stay on top of all other windows? If it does, this means "on top" is working correctly.
If you turn "on top" off and then try to switch to other applications, do other windows cover the mIRC window? It they do, this means "on top" has been turned off correctly.
At this point, if your Windows task bar is still being covered by mIRC, something else is going on that is not related to mIRC.
You could try changing your taskbar settings
, or moving the taskbar around. These seem to resolve the issue for some Windows users.
You could also try logging out of, or restarting, Windows.