Similar to my post on DNS Settings for the iPad, it is possible to change WiFi settings on the Apple TV so you can access US / American Netflix which has far more content than Irish / Canadian Netflix.
Do this by going into Settings > General > Network on the Apple TV and change the DNS setting from Auto to Manual. Insert 18.104.22.168 as the new DNS.
When configured correctly it should look like this:
A similar technique can be used to get US Netflix to work on iPhone, iPad, Sony PS3, Google Nexus 7, Smartphones and computers.
Note: This can make websites and apps think you are in America which may have an adverse (or positive) effect on your other apps, so you may wish to change these settings back when not using Netflix.
Check out my updated post: Here