See below for a round up of opening and closing arrangements in all the major libraries and archives across Ireland and in London:
Northern Ireland
The Public Record Office of Northern Ireland (PRONI) closed Monday 24 December to Wednesday 26 December, inclusive. Open Thursday and Friday 27-28 December and Monday 31 December (closing each day at 4:45pm). Closed Tuesday 1 January 2019. Open for normal hours from Wednesday 2 December (but note that late-night Thursdays do not recommence until 10 January).
North of Ireland Family History Society Library closed from 4 December until 8 January 2019.
The Presbyterian Historical Society of Ireland's Library and Archive will be closed from Thursday 20 December to Wednesday 2 January inclusive. Reopens on Thursday 3 January 2019.
Republic of Ireland
National Library of Ireland: The Genealogy Advisory Service will closed at 4:45 on Friday 21 December and reopen at 9:30am on Wednesday 2 January 2019.
The Reading Rooms will close for Christmas on Saturday 22 December at 12:45pm and reopen on Wednesday 2 January 2019 at 9:30am.
Exhibitions will be open 27-31 December inclusive from 1-5 pm.
All library buildings will be closed 24-26 December 2018 inclusive and again on Tuesday 1 January 2019.
The RCB Library will close at 1pm on Friday 21 December, and re–open for normal hours from Wednesday 2 January 2019.
Irish Architectural Archive will be closed from Noon on Friday 21 December and will reopen at 10am on Wednesday 2 January 2019.
All Dublin Public Libraries will be closed Monday 24 December to Tuesday 1 January 2019 inclusive. All libraries reopen on Wednesday 2 January 2019.
National Archives of Ireland will close at 12:30pm on Monday 24 December and will re-open on Wednesday 2 January 2-19 at 9:15am.
An Daonchartlann, the Archive & Research Centre of the Genealogical Society of Ireland, closed from 19 December to 10:30am on Wednesday 9th January 2019
London
The National Archives, Kew. Closed Saturday 22 December to Wednesday 26 December inclusive. Closed on Tuesday 1 January 2019. Otherwise, working to normal days and hours.
Society of Genealogists. Open Saturday 22 December and Monday 24 December. Closed from Tuesday 25 December to Friday 28 December inclusive. Open on Saturday 29 December. Closed for New Year holiday and annual stocktake from Tuesday 1 January to Monday 7 January inclusive. Reopens Tuesday 8 January.
The Irish Genealogical Research Society Library (Saturdays only) at the Society of Genealogists: Open on both Saturday 22 and 29 December. Closed Saturday 5 January. Normal Saturday openings from 12 January.