If you install the Visual C++ 2008 redistributables mentioned
here, does that resolve the issue? This is one of the more common reasons why OpenSSL DLLs will not work after installation.
Other than that, if you run the following alias in mIRC, it should show you which DLLs mIRC has loaded, if any:
showssl {
echo $ssl
echo $sslready
echo $sslversion
echo $ssldll
echo $ssllibdll
}