/flash [-bNwNrN] [window] <text>
This flashes the specified mIRC window/icon with text in the titlebar but only if mIRC is not the active application.
The -bN switch makes mIRC beep N times.
The -wN switch makes mIRC play the Flash sound specified in the Event Beeps section N times.
The -rN switch makes mIRC repeat the flash N times.
In all cases, if N is not specified the flash/sound continues indefinitely.
Also, you can click the button in the top left corner of the window and select Flash from the menu.