This site can be searched for any townland in Ireland. It will fill in some detail about your WWI soldier's family - brothers, sisters, parents, occupations and religion.
This site holds all of the service records for WWI - Army, Navy and Air Force. You will be charged a fee.
BBC World War One
The BBC site is good for background material on the war, causes of the war, campaigns and battles, virtual tours of trenches.
Commonwealth War Graves Commission
Online database giving details of memorials to those who died in WWI.
This site contains a variety of primary documents concerning the war (including memoirs and diaries), along with battlefield tours and many other useful sections.
History of the 9th Battalion, Royal Irish Fusiliers
Many Monaghan men joined this battalion, which was recruited from UVF members in Armagh, Cavan and Monaghan.
Ireland and the Great War
RTÉ site includes voice recordings of Irish WWI soldiers describing their experiences.
Irish Times digital archive
This site can be searched free at the county library in Monaghan.
Irish War Memorials Project
This website presents an inventory of war memorials in Ireland. It includes photographs of each memorial, the text of all inscriptions, and details of the site of the memorial.
Poetry, photographs and sounds
This is an excellent site for photographs of the war and it also has sound recordings of WWI soldiers on many topics including gas attacks, deserters, shell-shock. There are also many WWI poems to be found here.
Presbyterian Church in Ireland Roll of Honour 1914-1919
This site contains extracts from the above publication, published by the Presbyterian Historical Society of Ireland.
Tracing Your Irish Military Ancestors
Waterford Museum's website has a great section on this topic.
If you want to find out if your soldier's family was Unionist, check to see if their name appears in the Ulster Covenant or Declaration, signed in 1912.