Hi Bryan,
I am more familiar with SAML 2.0 for ABAP systems, however I can try to help you out with this Java issue.
Based in the error message it seems that there is a configuration problem in the IdP end not in the Java SP end. This URL is the one being accessed "http://portal.mycompany.com:80/irj/portal", thus you have to make sure that the IdP is sending the SAML Response to this application path/URL.
Currently it seems that the IdP is sending the SAML Response to a different path/URL: "https://portal.mycompany.com/saml2/sp/acs".
Therefore, try to locate in the IdP side which are the paths configured to receive the SAML Responses, and make sure that "http://portal.mycompany.com:80/irj/portal" is configured there.
Cheers,
Filipe Santos
