The National Archives of Ireland has added Herbert Wood's Guide to the records deposited in the Public Record Office of Ireland to its website. Wood was the former Assistant Deputy Keeper of the Public Records and his guide was published in 1919. It records the collections held by the PROI at that time, many of which were subsequently lost in the Civil War in 1922.
For genealogists, this is going to be hard reading. I think I'll wait until a three-Shredded-Wheat day. For those with braver hearts than mine, here's the link to the 178-page Guide (pdf).