Install the Atlassian Connector for Eclipse, as described here. (Latest version requires at least Eclipse Kepler SR2)

When install finishes, let it restart Eclipse.

Comprehensive usage instructions are here. Simplified instructions below for adding Liferay JIRA to Eclipse.

Menu: "Window, Show View, Other..."

Select "Task Repositories", "OK".

Right-click, "Add Task Repository..."

Select "JIRA", "Next".

Server: https://issues.liferay.com

User ID, Password: your credentials

If you wish, check the box: "Save Password"

Button: "Validate Settings"

A dialog appears: "Select repository location"

Choose "Keep current location: https://issues.liferay.com", "OK".

A message should display at the window top area: "Authentication credentials are valid."

Button: "Finish".

A dialog appears: "Would you like to add a query?"

It's best if you already have one or more JIRA filters created, to narrow the issues you'd like to work with on Eclipse.

Choose "Yes".

A dialog appears: "Edit Query"

If you already have a preferred filter, choose it from "Use saved filter from the repository"

Otherwise you can start with "Predefined filter for selected project". Pick "PUBLIC - Liferay Portal Community Edition" and "Assigned to me".

Button: Finish.

Menu: "Window, Show View, Task List".

You should see the query you just created. Expand it and you'll see your JIRA issues. Double-click one of them to see full details.

Right-click the issue, "Activate".

Notice the Package Explorer view goes blank with a message: "Empty task context, unfocus or Alt+click"

Menu: "Navigate, Open Type..."

Type "SourceFormatter", select both "SourceFormatter" and "SourceFormatterTest", "OK".

Notice the Package Explorer view displays only the opened classes, and nothing else.

Visit a few methods in the two classes. Notice the visited methods are added to Package Explorer's context. As you continue work on the JIRA issue, this context gradually shows the subset of Portal code you're reading and editing for the task at hand, filtering out all else.

If you activate a different JIRA issue, the context will change. Every issue has its own context, and if you switch between issues, the visible classes and methods will automatically change.

(Unfortunately, Eclipse doesn't have a feature to associate a Git branch to a JIRA issue and automatically checkout it when the issue is activated. You will have to checkout the Git branch yourself.)

When you deactivate the JIRA issue, Package Explorer goes back to showing the complete Portal source code again. If you want to see all the code without deactivating the issue, open the Package Explorer view menu and deselect "Focus on Active Task".