I've managed to get FishEye (2.7.5) up and running and working with my Mercurial repositories. I've also got the Jira (4.4.1) integration working and when I browse to a project I see my Changesets in the "Source" panel.
What I don't know how to do is map changesets to issues. For example if I have a Jira issue "ABCD-2123" and a changeset "067768a5d..." in the mapped repository how do I associate them so that the changeset appears in the Activity -> Source tab for the issue?
I think I understand why this isn't possible. FishEye scans the commit messages looking for issue ID matches, if there's no matches then there's nothing to show in the source tab for the issue.
To make this work I'd need to alter history in the Mercurial repository using something like histedit  which has its risks.