Ancestry has added two browseable sets of World War One 'War Diaries' to its database.
WW1 War Diaries are not generally very descriptive. They provide only the details on day-to-day movements of individual military units. From them, you can find out where your ancestor was stationed on a particular day, which battles the unit was engaged in, the level of casualties etc.
They rarely give names of individual soldiers but the surviving pages can provide a snapshot of what your ancestor experienced.
The larger of the two record sets is the War Diaries, France, Belgium and Germany, 1914–1920 collection. It covers British and colonial units serving in theatres of operations between 1914 and 1920, including Russia, at home and in the colonies, as well as military missions and Armies of Occupation between 1919 and 1920.
The second of the collection is the WW1 War Diaries, Gallipoli and Dardanelles, 1914–1916. This is the first timet these records have been made available online.
There may be gaps in coverage within the diaries and, for some regiments or divisions, you'll find nothing. Many diaries did not survive the war.
Among the many regiments included in the collections are the Irish Guards, Princess Victoria's (Royal Irish Regiment), Royal Irish Rifles, Royal Munster Fusiliers, Royal Inniskilling Fusiliers and Royal Dublin Fusiliers.
There are also a few Irish divisions within 'non-Irish' regiments ie the Army Cycle Corps even had its own 10th (Irish) Division. (You learn something every day!)
The original material is held by The National Archives in Kew, London.
UPDATE 22 April: Ancestry has published an informative blogpost (here) that will give you more detail about these collections.