愛爾蘭王國（愛爾蘭語：Ríocht na hÉireann；英語：Kingdom of Ireland）先後是英格蘭王國及大不列顛王國的附庸国，从1542年存續至1800年。它由英格兰以及後續的大不列颠君主统治，并与該君主的其他領土建立了共主邦聯。这个王国名义上由国王或女王從都柏林城堡統治，並由國王任命的总督負責管理。它有自己的立法机构（爱尔兰议会）、贵族制度（爱尔兰贵族制度）、法律制度和国家教会（爱尔聖公会）。
延至亨利八世在位的時候，意欲為終止和王后凱瑟琳的婚姻，被教宗克勉七世拒絕。因此亨利八世宣布英格蘭脫離天主教會管轄，自立為「全球英格蘭教會首領(of the Church of England in Earth Supreme Head)」。但在1533年教宗將亨利八世逐出天主教會，亨利八世也不承認教宗對愛爾蘭的宗主權，因此英王來自教宗的愛爾蘭領主頭銜合法性懸疑，導致愛爾蘭的憲制危機。因此後來愛爾蘭國會，通過《1542愛爾蘭王位法》，宣布亨利為「愛爾蘭國王」。
^W. G. Perrin and Herbert S. Vaughan, 1922, "British Flags. Their Early History and their Development at Sea; with an Account of the Origin of the Flag as a National Device", Cambridge University Press: Cambridge, pp. 51–52:
The red saltire on white ground which represents Ireland in the Union flag had only an ephemeral existence as a separate flag. Originating as the arms of the powerful Geraldines, who from the time of Henry II held the predominant position among those whose presence in Ireland was due to the efforts of the English sovereigns to subjugate that country, it is not to be expected that the native Irish should ever have taken kindly to a badge that could only remind them of their servitude to a race with whom they had little in common, and the attempt to father this emblem upon St Patrick (who, it may be remarked, is not entitled to a cross – since he was not a martyr) has evoked no response from the Irish themselves.
The earliest evidence of the existence of the red flag known to the author occurs in a map of "Hirlandia" by John Goghe dated 1576 and now exhibited in the Public Record Office. The arms at the head of this map are the St George's cross impaled on the crowned harp, but the red saltire is prominent in the arms of the Earl of Kildare and the other Geraldine families placed over their respective spheres of influence. The red saltire flag is flown at the masthead of a ship, possibly an Irish pirate, which is engaged in action in the St George's Channel with another ship flying the St George's cross. The St George's flag flies upon Cornwall, Wales and Man, but the red saltire flag does not appear upon Ireland itself, though it is placed upon the adjacent Mulls of Galloway and Kintyre in Scotland. It is, however, to be found in the arms of Trinity College, Dublin (1591), in which the banners of St George and of this saltire surmount the turrets that flank the castle gateway.
The Graydon MS. Flag Book of 1686 which belonged to Pepys does not contain this flag, but give as the flag of Ireland (which, it may be noted, appears as an afterthought right at the end of the book) the green flag with St George's cross and the harp, illustrated in Plate X, fig. 3. The saltire flag is nevertheless given as "Pavillon d'Ierne" in the flags plates at the commencement of the Neptune François of 1693, whence it was copied into later flag collections.
Under the Commonwealth and Protectorate, when England and Scotland were represented in the Great and other Seals by their crosses, Ireland was invariably represented by the harp that was added to the English and Scottish crosses to form a flag of the three kingdoms. At the funeral of Cromwell the Great Standards of England and Scotland had the St George's and St Andrew's crosses in chief respectively, but the Great Standard of Ireland had in chief a red cross (not saltire) on a yellow field.
When the Order of St Patrick was instituted in 1783 the red saltire was taken for the badge of the Order, and since this emblem was of convenient form for introduction into the Union flag of England and Scotland it was chosen in forming the combined flag of England, Scotland and Ireland in 1801.
^Morley, Vincent,Irish opinion and the American Revolution, 1760–1783, Cambridge: Cambridge University Press: 4, 2002 [20 January 2012], （原始内容存档于2016-05-03）,Féach ár bpian le sé chéad bliain aige Gaill in éigean, gan rí dár rialadh de Ghaeil, mo chian, i ríoghacht Éireann. … (the above Gaelic sentence is translated a few lines later as:) Consider our torment for six hundred years by violent foreigners, with no king of the Gaels ruling us, my grief, in the kingdom of Ireland. … Here can be seen, in close association, expressions of religious loyalty to the pre-Reformation faith represented by Creggan churchyard; dynastic loyalty to the house of Stuart; and national loyalty to 'ríocht Éireann', 'the kingdom of Ireland'.
^Richard Mant, History of the Church of Ireland, from the Reformation to the Revolution, London: Parker: 275, 1840,The enactments concerns the Church in Queen Elizabeth's first Parliament had no unpleasant effects upon its governors; save that by the Act of Supremacy, or rather their own obnoxious conduct in defiance of it, two bishops were deprived of their sees: Leverious, bishop of Kildare, who refused to take the Oath of Supremacy; and Walsh, bishop of Meath, who not only refused to take the oath, but preached also against the queen's supremacy, and against the Book of Common Prayer.
de Beaumont, Gustave and William Cooke Taylor,Ireland Social, Political, and Religious :Translated by William Cooke Taylor : Contributor Tom Garvin, Andreas Hess: Harvard University Press : 2006 :ISBN 978-0-674-02165-5 (reprint of 1839 original)
Pawlisch, Hans S., :Sir John Davies and the Conquest of Ireland: A Study in Legal Imperialism :Cambridge University Press, 2002 :ISBN 978-0-521-52657-9
Keating, Geoffrey :The History of Ireland, from the Earliest Period to the English Invasion (Foras Feasa Ar Eirinn) Translated by John O'Mahony 1866Full text at Google Books （页面存档备份，存于互联网档案馆）
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