Ireland
Helpful Timeline for genealogy and DNA interpretation:
Ireland Online
Maps
Placenames.ni.org - the history behind Northern Ireland place names
Ireland Online Genealogy Records (Guide from FamilySearch)
Emerald Ancestors - a subscription-only site focused on Northern Ireland - they do have some free resources
Guide to Ireland research from FamilySearch Wiki
Irish Ancestors Research Wizard - at The Irish Times is a very good place to start to tell what you are up against. Answer questions as best you can and they will give research suggestions
Irish Genealogy - many good sites are scattered around the internet. This site from the Department of Arts, Heritage and the Gaeltacht goes out to those sites and looks for you.
Genuki: Ireland - similar to the U.S. GenWeb Project - each county has a page.
Ulster Historical Foundation / Ancestry Ireland - has both free and subscription-based databases
Immigration
Irish Emigration Database - primarily for the Famine years 1846-1851 but some up through the 1890s and includes greater U.K. immigrants.
U.S. Ports of Arrival and their Available Passenger Lists 1820-1957 includes Canadian and Mexican Border Crossing Records and is laid out by state and helps you think about all possibilities and what might still exist though some are difficult to get to.
If you have Ancestry and have famine immigrants don't miss
New York, Irish Immigrant Arrival Records 1846-1851
but also be aware that there were other ports AND many came in through Canada
Database of Advertisements for Irish Immigrants in the Boston Pilot from Boston College
Maps
Newspapers