This is a pretty simple process. I have only tested it with Windows 8 Pro/Enterprise. It is not clear to me if Windows 8 Core has the same functionality. The Irish Language Pack for Windows 8 RT is not yet available.
From the start screen begin typing the word “language”. Then click on settings. In the second column of search results click on the Language result as show in the image.
The Add a Language dialog will appear as in this image:
You will then be sent to the Change your language preferences page. Click options next to Gaeilge.
Then click Make this the primary language.
You will have to log off and log back on to your computer for the changes to take effect.
There you have it. Your PC will now use Irish as its display language.