The Great Seal of Ireland was the seal used until 1922 by the Dublin Castle administration to authenticate important state documents in Ireland, in the same manner as the Great Seal of the Realm in England. The Great Seal of Ireland was used in the Lordship of Ireland (1180s–1534) and the Kingdom of Ireland (1534–1800), and remained in use when the island was part of the United Kingdom of Great Britain and Ireland (1801–1922), just as the Great Seal of Scotland remained in use after the Act of Union 1707.
The office of "Lord Keeper of the Great Seal of Ireland" was held by the Lord Chancellor of Ireland, and the physical seal was kept in the Court of Chancery. When the Chancellor was absent, Lords Commissioners of the Great Seal were appointed. The seal was affixed to documents issued by the Privy Council of Ireland and its head the chief governor (latterly called the Lord Lieutenant). In the fifteenth century, the Chief Governor was generally non-resident and was represented by a Lord Deputy. The Governor or Deputy would issue a type of writ called a fiant to the Lord Chancellor, mandating the issue of a patent ("letters patent") under the Great Seal. In the fourteenth century, the Chancellor was entitled to a guard of six men-at-arms and twelve mounted archers, in part to protect the seal in his custody. The Chief Governor was appointed in London under the Great Seal of England, but a 1498 Act allowed a vacancy to be temporarily filled by the Dublin administration under the Irish seal. This practice was applied several times in the 1690s. In the fifteenth century, the Governor was appointed under the king's privy seal and appointed his Deputy under the Irish seal. From 1700 to 1767, non-resident Lords Lieutenant were appointed under the British Great Seal, and would in turn use the Irish seal to appoint resident Lord Justices as deputies.
Before the Act of Union 1800, any bill passed by the Parliament of Ireland and approved by the Privy Council of Ireland was sent to London under the Great Seal of Ireland and, if approved by the Privy Council of England (later the Privy Council of Great Britain), was returned under the Great Seal of the Realm to be enacted. This procedure was required both under Poynings' Law (1495) and under the Constitution of 1782 which amended it. Parliament was summoned under the Great Seal of England rather than that of Ireland, even after 1782.
Titles in the Peerage of Ireland were originally created under the English seal. After the Williamite War they were usually created under the Irish seal, but creations under the British seal continued, even after the Constitution of 1782, until the Act of Union 1800. Robert Raymond, 1st Baron Raymond wrote that, under the British seal, the Irish nature of the peerage had to be made explicit. Sometimes a single patent created separate titles in the Irish and English/British peerages for the same person. After the Union, the question of whether Irish peers appointed under the British seal were entitled to vote for Irish representative peers was considered by the British House of Lords in 1805.
The 1289 "ordinance for the State of Ireland" forbade purveyance except by a commission under the Great Seal of Ireland. From the Tudor Reconquest, appointments to the Irish judiciary were made entirely under the Irish seal.
An Irish chancery was established in 1232 separate from the English chancery, and all documents issued from the Irish chancery were sealed with the "great seal of the king used in Ireland". Most thirteenth-century land grants continued to be issued in England with the English seal, and then sent to the Irish chancery to be enrolled. In 1256, King Henry III granted the Lordship of Ireland to his heir, the future Edward I, and ordered that Edward's personal seal should have "royal authority" there. Henry took Ireland back from Edward in 1258. Robert de Vere, Duke of Ireland was similarly authorised to use his own great seal by Richard II in 1385; Richard ordered de Vere's seal to be broken in 1389. In the 1300s, pardons for felony were granted under both the Irish and the English seals.
In 1417, the Chancellor, Laurence Merbury, refused to authenticate with the seal a petition to the King from the Parliament on the state of Ireland. In 1423, the Chancellor Richard Talbot, archbishop of Dublin, refused to acknowledge Edward Dantsey, Bishop of Meath, as Deputy because the Governor, Edmund Mortimer, 5th Earl of March, had appointed Dantsey with his privy seal instead of the Great Seal. In 1460, Richard of York, under threat of arrest in England, fled to Ireland and persuaded the parliament to pass a declaration of independence stating in part that Ireland was "corporate of itself" and that "henceforth no person or persons being in the said land of Ireland shall be, by any command given or made under any other seal than the said seal of the said land, compelled to answer to any appeal or any other matter out of the said land". The Lord Deputy Gerald FitzGerald, 8th Earl of Kildare refused Edward IV's command to annul this, and it was not until Edward Poynings in 1495 that this was done.
In 1478, after Kildare was replaced as Lord Deputy by Henry Grey, 4th (7th) Baron Grey of Codnor, the Lord Chancellor Rowland FitzEustace, 1st Baron Portlester, who was Kildare's father-in-law, organised a campaign of non-co-operation with the new Deputy and refused to hand over the Great Seal, making the conduct of official business impossible. Kildare argued that Grey had been appointed under the privy signet instead of the Irish Great Seal. King Edward IV ordered Thomas Archbold, the Master of the Royal Mint in Ireland, to strike a new Great Seal, "as near as he could to the pattern and fabric of the other, with the difference of a rose in every part". The king decreed that the Seal held by Portlester was annulled, and that all acts passed under it under it were utterly void; but to no avail. So effective was the campaign of obstruction that after a few months Lord Grey was forced to return to England. In 1484, James Collynge, a clerk, was brought before the Chief Justice of Common Pleas charged with forging the Great Seal of Ireland.
One of the developments which Henry VII and the Tudor Reconquest of Ireland sought to reverse was that the Lord Deputy's private seal, kept by his secretary, was being used in place of the Great Seal of Ireland.
In 1662, the fee to the Lord Chancellor for patents under the Great Seal was increased to 10 shillings sterling per patentee. The 1722 patent for Wood's halfpence was issued under the British seal rather than the Irish, which was among the complaints Jonathan Swift made in his Drapier's Letters condemning the currency.
The Act of Union 1800 provided that the Great Seal of Ireland could continue to be used in Ireland, and that at elections to the Westminster parliament for constituencies in Ireland, the writs and certified returns would be under the Irish rather than the British seal.
A new seal was created for each new monarch, whose likeness would be on the obverse. (On the accession of Richard II in 1377, the seal of Edward III was re-used to save money, with only the king's name being updated.) The seal included the arms of Ireland: until 1536, three crowns; after that, an Irish harp. From Queen Victoria on, the Great Seal of Ireland had the same design as the Great Seal of the Realm except for the inclusion of the Irish harp. The seal matrix was cast in silver and the impressions made in sealing wax. An unused sketch by Nicholas Hilliard for a Great Seal for Elizabeth I is on display in the British Museum. Irish Great Seals are attested from the thirteenth century, though surviving impressions of them are rare. Most state papers were destroyed, in multiple fires between 1304 and 1758, and in an explosion in the Battle of Dublin in 1922. According to Hilary Jenkinson, "a fairly intensive search some years [before 1954] in England and Ireland for impressions of the Irish Seals produced a total of only forty, for the period from the thirteenth century onward".
The Government of Ireland Act 1920 transferred custody of the seal from the Lord Chancellor to the Lord Lieutenant on 27 June 1921. The physical seal was in the Crown and Hanaper Office in the Four Courts when that was occupied by the Irish Republican Army on 14 April 1922 in the buildup to the Civil War. Under British law the writs for the Irish Free State election of 16 June 1922 had to be passed under the Great Seal, so an order in council was passed to authorise a substitute seal. The explosion in the Four Courts during the Battle of Dublin was initially assumed to have destroyed the Great Seal, but it was later found in the rubble. The 1920 act intended the Great Seal of Ireland to be used by both Northern Ireland and Southern Ireland, and several writs for the Westminster election of 15 November 1922 were burnt by republicans in Dublin when sent from Northern Ireland to be sealed. Under the Irish Free State (Consequential Provisions) Act 1922 a separate Great Seal of Northern Ireland was created. The Great Seal of the Irish Free State was adopted in 1925 by the Irish Free State, prior to which the Governor-General used his "Private Seal". As in other Dominions of the then British Commonwealth, the Great Seal of the Realm was used for some diplomatic functions. In 1931, a separate "External Great Seal" was struck to replace the Great Seal of the Realm in these functions. Under the 1937 Constitution of Ireland, the Seal of the President of Ireland replaced the internal Free State seal, but the External Great Seal remained in use, until the Republic of Ireland Act 1948 came into force.