Most examples on how to do this are for Linux. On FreeBSD the process is only slightly different:
Install the security/ca_root_nss port
Configure irssi by running the following commands in it:
/network add -nick 'nick' -realname 'real name' freenode /server add -auto -ssl_verify -ssl_cafile /usr/local/share/certs/ca-root-nss.crt -network freenode chat.freenode.net 7000 /sav
As you can see, the only things to note are the package you need and the changed path to the certificates. Unlike Linux, FreeBSD collects certificates in a single file and stores them in /usr/local/share/certs/ca-root-nss.crt instead of in /etc/ssl/.