Here are a few sites I’ve found useful for research:
The National Library of Ireland
The premier source for books, manuscripts, newspapers and personal papers relating to Irish history. The catalogue is online, but you’ll have to come to Dublin to view the material.
The National Archives (Ireland)
A mine of information - particularly useful now that the Bureau of Military History witness statements are available online. Currently hosting a fascinating online exhibit about the Anglo Irish Treaty (which features in Book III - The Soldier’s Farewell).
The National Archives (UK)
For British military service records, medal cards and other useful information.
The Royal Dublin Fusiliers Association
A website dedicated to commemorating the men of the Royal Dublin Fusiliers (Stephen Ryan’s regiment in The Soldier’s Song trilogy).
A useful website for general information pertaining to the First World War. Some fascinating photographs and biographical information here.