1. FamilySearch
Irish civil registration indexes 1845-1958, plus parish records of births (baptisms), marriages and deaths have been transcribed by the Church of Jesus Christ of Latter-day Saints and can be searched for free on their Web site at FamilySearch.org. Browse to "Ireland" and then search each database directly for best results. Coverage is by no means complete, but it is a good place to start. Another search trick is to use Ireland IGI Batch Numbers to search the International Genealogical Index - see Using IGI Batch Numbers for a tutorial. Free
2. Irish Family History Foundation
The Irish Family History Foundation (IFHF) is a coordinating body for a network of government approved genealogical research centres in the Republic of Ireland and Northern Ireland. Together these research centres have computerized almost 40 million Irish ancestral records, primarily church records of baptisms, marriages, and burials, and made the indexes available online for free. To view a detailed record you can purchase credit online for instant access at a per-record cost. Free index searches, pay to view detailed records
3. Irish Origins
This subscription-based genealogy site features an Irish Wills Index (1484-1858), the 1851 Dublin City Census, Irish Royal Garrison Artillery Records, and the best online version of Griffiths Valuation. While other Web sites such as MyIrishAncestry.com offer a free index to Griffiths, Irish Origins offers full details and links to digitized images. Free name search. Subscription
4. National Archives of Ireland
The genealogy section of the National Archives of Ireland offers several free searchable databases, such as the Ireland-Australia Transportation Database, along with finding aids to many useful record series held in the National Archives. Of special interest is their digitization of Irish 1901 and 1911 census records which are complete an available online for free access. Free
5. Emerald Ancestors
This extensive Ulster genealogy database contains baptism, marriage, death, burial, and census records for over 1 million Irish ancestors in Counties Antrim, Armagh, Down, Fermanagh, Londonderry and Tyrone. Most database results are indexes or partial transcriptions. Subscription
6. Ancestry.com - Irish Collection, 1824-1910
This subscription-based site offers access to several important Irish collections, including Griffiths Valuation (1847-1864), Tithe Applotment Books (1824-1837), Ordnancy Survey Maps (1824-1846) and the Lawrence Collection of Irish Photographs (1870-1910). Subscription.

