He was a prolific Irish scholar and church leader, who today is most famous for his identification of the genuine letters of the church father, Ignatius of Antioch, and for his chronology that sought to establish the time and date of the creation as "the entrance of the night preceding the 23rd day of October... the year before Christ 4004"; that is, around 6 pm on 22 October 4004 BC, per the proleptic Julian calendar.

It is recorded in Alfred Webb's, A Compendium of Irish Biography (1878) that his father, a clerk in the court of Chancery, was said to have been descended from one, Neville, who came over (to Ireland) with King John in the capacity of usher and had changed his name to that of his office.

A noted collector of Irish manuscripts, he made them available for research to fellow scholars such as his friend, Sir James Ware.

Tension was rising between England and Spain, and to secure Ireland Charles I offered Irish Catholics a series of concessions, including religious toleration, known as The Graces, in exchange for money for the upkeep of the army.

Ussher was a convinced Calvinist and viewed with dismay the possibility that people he regarded as papists might achieve any sort of power.

This begins: The religion of the papists is superstitious and idolatrous; their faith and doctrine erroneous and heretical; their church in respect of both, apostatical; to give them, therefore, a toleration, or to consent that they may freely exercise their religion, and profess their faith and doctrine, is a grievous sin.The Judgement was not published until it was read out at the end of a series of sermons against the Graces given at Dublin in April 1627.

Following Thomas Wentworth's attainder in April 1641, King Charles and the Privy Council of England instructed the Irish Lords Justices on 3 May 1641 to publish the required Bills to enact the Graces.

In 1633, Ussher wrote to the new Archbishop of Canterbury, William Laud, in an effort to gain support for the imposition of recusancy fines on Irish Catholics.

Laud did that, rewriting the charter and statutes to limit the authority of the fellows, and ensure that the appointment of the provost was under royal control.

In 1634, he imposed on the college an Arminian provost, William Chappell, whose theological views, and peremptory style of government, were antithetical to everything for which Ussher stood.

William M. Abbott, Associate Professor of History at Fairfield University, argues that he was an effective and politically important bishop and archbishop.

[8] The story that he successfully opposed attempts to reintroduce the Irish language for use in church services by William Bedell, the Bishop of Kilmore, has been refuted.

Ussher had an obsession with "Jesuits disguised as" Covenanters in Scotland, highwaymen when he was robbed, non-conformists in England, it was a remarkable list.

[11] However, Ussher also wrote extensively on theology,[12] patristics and ecclesiastical history, and these subjects gradually displaced his anti-Catholic work.

In 1639, he published the most substantial history of Christianity in Britain to that date, Britannicarum ecclesiarum antiquitates – the antiquities of the British churches.

Ussher was very reluctant to arrive at firm judgements as to the sources' authenticity – hence his devotion of a whole chapter to the imaginative but invented stories of King Lucius and the creation of a Christian episcopate in Britain.

In the years before the Wars of the Three Kingdoms, his reputation as a scholar and his moderate Calvinism meant that his opinion was sought by both King and Parliament.

Despite their occasional differences, he remained a loyal friend to Thomas Wentworth, 1st Earl of Strafford, and when the latter was sentenced to death by Parliament, pleaded with the King not to allow the execution of the verdict: unlike some of his episcopal colleagues, he insisted that the King was absolutely bound in conscience by his promise to Strafford that whatever happened his life would be spared.

The King did not take his advice, but clearly afterwards regretted not doing so, as is shown by his reference on the scaffold to Strafford's death as "that unjust sentence which I suffered to take effect".

Ussher researched and found a shorter set, usually called the Middle Recension, and argued that only the letters contained in it were authentically Ignatius's.

Ussher's work is now used to support Young Earth Creationism, which holds that the universe was created thousands of years ago (rather than billions).

[17] In 1655, Ussher published his last book, De Graeca Septuaginta Interpretum Versione, the first serious examination of the Septuagint, discussing its accuracy as compared with the Hebrew text of the Old Testament.