JIRA 4 Integration Plug-in
This is a “version 1” JIRA plugin, meaning that you should deploy it into WEB-INFlib directory of your JIRA installation. (See Managing JIRA plugins for details.)
Also, make sure to enable JIRA RPC plugin to invoke JIRA operations remotely.
If you’re not sure what else you should do to enable JIRA integration on YouTrack side, see relevant items in YouTrack FAQ.