AncestryIreland.com is the website of the Ulster Historical Foundation, a not-for-profit organisation. It’s a massive website absolutely full of useful information and resources.
The Ulster Historical Foundation is a long-established, highly reputable research and publishing agency. It offers extensive knowledge on the sources available for tracing Irish and Scots-Irish ancestors. Services include online databases of over 2 million records, genealogy and history books, and personal ancestral research. Supporters belong to its Ulster Genealogical & Historical Guild, where members can share research interests and receive two annual publications, invitations to educational and social events, and product discounts. By making an appointment, members of the public can use its databases at its office, as well as a consultation on your family history research.
You need to be a member of the foundation to use their databases. (Prices are listed in the right-hand column of their website.)