For those who search for a way to render wiki markup in a Velocity file, for example in JIRA email templates:
#set ($rendererManager = $componentManager.getRendererManager() )
#set ($renderer = $rendererManager.getRendererForType("atlassian-wiki-renderer"))
#set ($ircontext = $renderer.getRenderContext() )
#set ($renderedText = $renderer.render($wikimarkuptext, $ircontext) )
Caveats:
Not all characters will be displayed by Outlook in the email. Bold and URL links worked for me, but not tables or emoticons.
Also, I'm not sure if the ComponentManager object is still accessible in the email template Velocity context for newer versions of JIRA