The recommended way to link TestRail test cases to Jira is to use the TestRail-Jira integration. This will automatically push TestRail test case results to Jira, and will also allow you to view and create TestRail test cases from within Jira.
If you're not using the integration, you can still link TestRail test cases to Jira manually. To do this, simply create a custom field in TestRail (e.g. "Jira ID") and enter the corresponding Jira issue ID for each test case.