This time it's aimed at Migration Records, so you get free access to collections such as passenger records, naturalisation records, border crossing records, crew lists, settler and convict lists etc. See the full list of featured collections here.
Free access will run from now until 11:59pm (Dublin/London time) on Monday 4 October.
You'll need to be registered with Ancestry to gain the free access. If you're not already registered, follow the link and do submit your name and email address.
Ancestry will then send you a user-name and password. Full terms and conditions are on the site.