1: Log in to webmail. This can be done by opening a new tab in your browser and typing your domain in the URL bar, followed by /webmail (mydomain.ie/webmail for example)
2: Once logged in to Roundcube, click Settings
3: Once in Settings, click Identities
4: Select the email address you want to add your signature to
5: At the bottom, you will see a Signature box. You can add a plain text signature here. If you want to add an HTML signature, you can click the small picture icon on the top left of this box to enable HTML
6: Click Save