When you visit the Virgin Media website, you’ll see a link in the top right of the landing page next to the search field. Just click it, and you’ll be brought to the log in page. You can also access it by going to https://www.virginmedia.ie/webmail.
After you’ve arrived at Webmail log in you enter your full UPC/Virgin Media Webmail address and its password. When you’ve done that, click "Sign in" and off you go.