You can either link test cases/results (via References) or test results (via Defects). Test results in JIRA are shown as follows:
Linking entire test runs/plans is not currently possible but it would usually be too overwhelming to link a single JIRA issue to a entire run or plan with hundreds or more of tests or results. Once you've linked a case or result to an issue, this will also include details about the test run/plan/milestone and you can quickly jump to TestRail to see all run/plan/milestone details then:
(see the links above the result comment)
I hope this helps!