To open External link via SP.UI.ModalDialog you can use the following code

<a href="#" onclick="openInDialog(500,600,true,true
,false,'http://www.SharePoint.com');">MS SharePoint Site</a>

function openInDialog(dlgWidth, dlgHeight, dlgAllowMaximize, dlgShowClose, pageUrl)
{
var options= { width: lgWidth, height: dlgHeight, allowMaxize: dlgAllowMaximize,
showClose: dlgShowClose, url: pageUrl};

SP.UI.ModalDialog.showModalDialog(options);

}

For more details check the main reference

If you need to use Jquery you can also use Fancy box as Iframe, I think this link may help you to use fancy box inside SharePoint

First of all, you should be aware of if the publisher doesn't allow to display its content in Iframe, you will not be able to show it inside SharePoint.

Otherwise, you will be able to open an External URL using SP.UI.ModalDialog SharePoint Modal Dialog.

Please, check the details at Open a link via Modal Dialog in SharePoint

[Update]

In case, you would like to use Jquery, so you can use Fancy box as Iframe, I think this link may help you to Use JQuery to show Modal Popup Per Session In SharePoint