I have run the Language Support tool and clicked Install/Remove Languages... and installed Irish. I can see from the Ubuntu Software Center that at least two packages are installed *-ga-base being one of them.
But Irish still doesn't appear on the list in the Language Support tool. Apparently, when I installed the language, there is supposed to be a + for more options, which is missing (and is a bug). (See the accepted answer here How to change system language? for more details)
How can I get languages other than just English on the list in Language Support?
Update: Running gnome-language-selector from the terminal I see this output:
gnome-language-selector:22702): WARNING **: Couldn't connect to accessibility bus: Failed to connect to socket /tmp/dbus-iIuzZfldLm: Connection refused  [<Package: name:'firefox-locale-ga' architecture='amd64' id:2712>, <Package: name:'language-pack-ga-base' architecture='amd64' id:2713>, <Package: name:'language-pack-gnome-ga-base' architecture='amd64' id:5653>, <Package: name:'language-pack-gnome-ga' architecture='amd64' id:5651>, <Package: name:'myspell-ga' architecture='amd64' id:1918>, <Package: name:'libreoffice-l10n-ga' architecture='amd64' id:11807>, <Package: name:'kde-l10n-ga' architecture='amd64' id:33666>, <Package: name:'language-pack-ga' architecture='amd64' id:5649>]
$ /usr/share/language-tools/language-options en en_AU en_CA en_GB en_NZ en_US
Output of locale -a:
$ locale -a C C.UTF-8 en_AG en_AG.utf8 en_AU.utf8 en_BW.utf8 en_CA.utf8 en_DK.utf8 en_GB.utf8 en_HK.utf8 en_IE.utf8 en_IN en_IN.utf8 en_NG en_NG.utf8 en_NZ.utf8 en_PH.utf8 en_SG.utf8 en_US.utf8 en_ZA.utf8 en_ZM en_ZM.utf8 en_ZW.utf8 POSIX
Running sudo apt-get install --reinstall language-pack-ga-base
Setting up language-pack-ga-base (1:14.04+20150804) ... Generating locales... ga_IE.UTF-8... up-to-date Generation complete.
And subsequently (I may have been ssh'd into the wrong machine first time!)
Generating locales... ga_IE.UTF-8... cannot open locale definition file `ga_IE': No such file or directory failed Generation complete.