The "Name" field ("alternate name"/"real name"/etc, as discussed in T798), is currently the only alternate way to refer to someone. So, if someone uses a different name onwiki and onIRC, it is impossible to discover who they are.
E.g. https://phabricator.wikimedia.org/p/demon/ is currently going by the username "demon" and the alt-name "Chad Horohoe". He lists his IRC name as "ostriches" and his mediawiki-username as "😂" but neither of those names successfully autocomplete if I type them into the Subscribers field.
E.g.2. ditto for https://phabricator.wikimedia.org/p/Rdicerb/ and her IRC name "rdaiccherlb".
E.g.3. ditto for https://phabricator.wikimedia.org/p/jrbs/ and his IRC/LDAP name "foks"
Upstream report: https://secure.phabricator.com/T4267#47083