1From 1642, overlapping control with Confederate Ireland. 2See notes regarding use of a crowned harp as the arms of Ireland. No official flag is known to exist for the Kingdom of Ireland. Numerous unofficial flags were used throughout its history, including: 1. Azure, a harp Or, stringed Argent, based on the coat of arms adopted in 1541 and much later to become the presidential standard; 2. Vert, a harp Or, stringed Argent, the Leinster flag, used from the mid-17th century; and 3. Argent a saltire Gules, Saint Patrick's Flag, from 1783. The latter was integrated into the Union Flag, the first flag officially used to represent Ireland. However, the second appears to have been the most popular and its use as a naval jack is debated as to whether it had official status or not. 3 Represented by a Lord Lieutenant of Ireland. 4Northern Ireland.
^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 saltire 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 satire 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 representative 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.
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 9780674021655 (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 9780521526579