How do I get a Gaelic UI in OS 10.6?

Have you used any methods to remove additional languages, or use only one during the original install?

If so, you may need to go back to the SL DVD and do a custom install - that or use a program named Pacifist to fetch them accordingly.

I see Gaeilge (Irish Gaelic when I mouse over that choice) in the 'Edit List...' options, so be sure you've added it.

In the mean time if you think the language you want is already installed, try deleting the com.apple.systempreferences.plist

~/Library/Preferences...

And be sure to enable the specific keyboard input source you want in that panel.

I would also prefer the gaelic script, instead of the default latin one.

The tech specs for OS X lists the 18 languages in which the UI is localized:

http://www.apple.com/macosx/specs.html

Unfortunately gaelic is not yet among them. You can ask Apple for this here:

http://www.apple.com/feedback/macosx.html

Putting gaelic at the top of the list in system prefs/language & text/languages cannot affect the OS UI, but will ensure that any app you install which does include a gaelic localization will display that way, and should also make gaelic the the default spell checker (if you install one) and make files sort in gaelic sort order.

I wonder if there are any 3rd-party fixes for this...

Sometimes individuals have created additional localizations of OS X, but I think it is a big job. You might ask Apple Ireland staff if they have ever heard of someone in that country working on one.

In any case I think it would have to be in the ordinary Latin script supported by Apple system fonts, rather than the Gaelic script variant.