Given it's primary area of focus – the states of Maine, Vermont, New Hampshire, Massachusetts, Connecticut, Rhode Island and the neighbouring state of New York, where so many Irish emigrants arrived and settled – this is an important collection for Irish family history researchers, whether their family is still in Ireland or long settled in the USA.
One of the most useful for tracking down extended family members, in my experience from the European side of the Atlantic, is the Massachusetts RC Archidiocese of Boston's records, 1789-1900. (I just checked the indexed database was included in the free access but only the browse version appeared in the list; however, the indexed version is included... here's the direct link: MA-RC).But there's a lot more besides. US Census records, wills, church records, tax lists, burial registers, immigration and naturalization records and so on.
The database isn't limited to the northeastern United States, either. It holds record sets for California, Iowa, West Virginia and other states, as well as a good many from England, Scotland, Wales, Sweden, Costa Rica, Australia... you get the picture.
Free access is now open and will be available for a full seven days until 11:59pm EDT on Tuesday 19 November (5am GMT on 20th).
You'll need a free guest account to explore the database.
Click the image for the home page, and enjoy yourselves!