Windows as Gaeilge
For the Linux users amongst us.
I'm using Fedora 22, with Gnome3 configured to be as Gaeilge. Works so well, that even things in the command line interface start spewing messages in Irish.
It's under settings->Region and Language.
Also, if you set up the keyboard there to be English (International AltGr dead keys) you can use the right hand AltGr key to put fada's into your text quite quickly.
I'm guessing there is going to be something quite similar for KDE and Cinnamon based UI's (though I haven't checked).
Seeing as Fedora likes to put all of it's features upstream, it's quite likely that this language support is also in any other distribution you'll try (no guarantees though).
I think that just adds keyboard support - I have Gaeilge added as a language, so that I can switch keyboards to type fadas, but it doesn't change the system language for Windows.