I tried various instructions before for getting the Firefox browser to display its user interface in Irish, but it never seemed to take hold. Anyway, I'm using Firefox , and finally got it to work. I just did this:
1. Clicked on "ga-IE.xpi" from
http://releases.mozilla.org/pub/mozilla.org/firefox/releases//mac/xpi/ (if you're on Windows, get it from here:)
http://releases.mozilla.org/pub/mozilla.org/firefox/releases//win32/xpi/ and it asked me to install it. I said yes.
2. Shut down and restarted Firefox.
3. Selected Tools > Language > Irish (Ireland) from the menu.
4. Shut down and restarted Firefox.
5. Ta da! It's all "as Gaeilge anois".
Note that I had previously installed the Locale Switcher Extension from
http://benjamin.smedbergs.us/switch-locales/ I'm not sure if it's still needed. If you don't see a Language item on your Tools menu, you might have to install that first.