The BHO, created in 2003 by the Institute of Historical Research and the History of Parliament Trust, offers a digital collection of 1,280 key printed primary and secondary sources for the history of Britain and Ireland, with a special focus on the period 1300 to 1800. Under normal circumstances, some 1,000 of these sources are free for individual researchers to study while about 200 volumes – all transcribed – are accessible only with a subscription. The latter includes the Calendar of State Papers, Ireland, covering the period 1586-1606, in 13 volumes.
From today, all of the BHO content can be freely accessed. A detailed blogpost from the editorial team provides more information about the content now available.
(If you dive straight in to the search box, you might miss an important statement on the home page explaining that the "search within volume" box is currently not working properly. The BHO tech team is working on a fix.)
UPDATE, 4 June 2020: The period of free access has been extended to the end of September.