To celebrate St Patrick's Day, Ancestry is offering free access to their collections of passenger lists, border crossings and passports this weekend (presumably till midnight US time on Sunday).
Search Passenger Lists.
Search Border Crossings and Passports.
There's also a video 'Tracing Your Irish Ancestors Back to the Homeland'.
If you're interested in migrants, come to our Migration themed Conference on 11th May 2013 in Galashiels, Scotland.