Daniel O’Neill was born in or around present day Belfast he was descended on both sides from the mighty O’Neill clan, both Tyrone and Clandeboy lines ran through his blood, of course the O’Neill Clan being the most powerful clan in Ulster and Ireland since the 5th century had provided many of its ‘high kings’. Daniel O'Neill was the eldest son of Con Mac Niall O'Neill, lord of Clandeboye and his wife, Eilis was a (a paternal niece of Hugh O’Neill, 2nd Earl of Tyrone).

His father, Conn O’Neill of Castlereagh, lost the family lands after defeat at the Kinsale, leaving Daniel to inherit a small estate at a young age in 1619. He then became a ward of Chancery and was raised in England as an Anglican. His Father Conn succeeded to a vast estate in county Down which he valued at £12,000 per year but an argument concerning rents due to him under his right as the O’Neill of Clandeboy with local English and Scottish settlers arose in about 1603. He and his wife were then swindled out of his family estates by a Scottish settler called Hugh Montgomery, or 1st Viscount Montgomery in order to obtain Conn’s pardon from the English government.

Daniel and his brother were granted an annuity of a mere £160 per year but he was according to English writers of the time ‘in subtlety and understanding much superior to the whole nation of the old Irish’, and was able ‘to support himself without dependence or beholdingness’.  Again the English writers of the time state To him having ‘a natural insinuation and address which made him acceptable in the best company’ he added ‘a courage very notorious’, spending the winters at the Court of Charles I and the summers on campaign in the Low Countries, ‘which was as good an education towards advancement in the world as that age knew, he also gained some experience in diplomacy.

With little prospects in his native Ireland, Daniel served under Lord Conway in the Low Countries as mentioned above during the 1630s, gaining military experience and friends such as Elizabeth of Bohemia and her husband, Frederick V, Elector Palatine. Using these connections, he petitioned for his lands to be restored to him, but despite support by William Laud, Lord Arundel, and Charles I Louis, Elector Palatine, he was rebuffed by the Lord Deputy Wentworth (later Earl of Stafford) But he failed to get the to dispossess the settlers from Clandeboy and Ards, and during the debates on his enemy’s attainder petitioned the Long Parliament.

Waiting for his petitions to be accepted, O'Neill returned to the Low Countries in 1637 and saw action at the Siege of Breda and later in the Bishops Wars, where he was captured at the Battle of Newburn and imprisoned at Newcastle upon Tyne.

After O'Neill's release, he conspired to overthrow Wentworth but the discovery of the subsequent plots he became involved in, forced him to flee to the continent in mid-June 1641. Hoping for immunity, he returned to England a few months later and surrendered to John Pym, but was sent to Gatehouse Prison to await his trial. His health began to suffer and in 1642, he was petitioned for better treatment and was transferred to the Tower of London where he escaped by tying Bed sheets and a tablecloth together and dressing as a woman he is one of only a few that survived the clutches of the Tower.

Fleeing to Brussels, O'Neill gathered troops and arms for the royalist campaigns in the English Civil War, served under Prince Rupert becoming his second-in-command of the Rhine and fought at Edgehill, Chalgrove Field and the First Battle of Newbury. He was then employed in negotiations with his own Irish kin, who were led by his maternal uncle, Owen Roe O’Neill, and he was offered the command of their army and the chieftainship of the clan if he would change his religion. But he refused, and joined Charles II in the Worcester campaign, Daniel went on to serve as a spy to the de jure Charles II at The Hague.

In the Interregnum he spent in royalist conspiracy and diplomacy (under the appropriate cover-name of ‘Subtle’). He paid two visits to England, and together with Ormonde and Bristol, accompanied Charles on his fruitless journey to Fuentarrabia in 1659. Following the restoration in 1660, Daniel was rewarded and appointed to money-making positions by Charles II, including: as a Groom of the Bedchamber, Captain in the Horse Guards, Member of Parliament for St Ives, admittance to Grays Inn, Mining rights, monopoly of the manufacture of gunpowder to the crown, warden of St James Palace, Postmaster General and accountant for the regulation of ale houses. He subsequently became one of the richest men in the kingdom. In 1662, he married his old friend, the Countess of Chesterfield and built Belsize Park for her.

He leased the manor of Belsize from the crown, and built a great house ‘at vast expense’. With the Earl of Cork he was given a customs post in trust for the Roman Catholic widow of Endymion Porter. He formed a syndicate with Francis Vaughan, Lord Vaughan, and others which was given the sole right for 41 years to prospect for minerals in Wales and the north of England. Together with Sir George Carteret he farmed the duty of 5s. per ton imposed as a reprisal on French shipping under the Navigation Act. His most lucrative boon was undoubtedly his monopoly of supplying gunpowder to the Ordnance, but he may also have hoped to re-establish his position in Ireland; his pension of £500 was confirmed, he obtained the reversion of two Irish estates forfeited in 1641, and he represented County Down in the Dublin Parliament.

At the general election of 1661 O’Neill was unsuccessfully recommended by the Earl of Portland for the Isle of Wight borough of Newtown. He was chiefly responsible for arranging for the return to England and the appointment to office of his old friend Sir Henry Bennet. In January 1662 he defeated Edward Nosworthy 1 at St. Ives in London with the assistance of Francis Godolphin, and obtained a letter from the King to the Exeter chapter urging them to transfer the reversion to two of their manors from Nosworthy, who had bought them ‘in the late times’, but whose actions had not been ‘such as to merit favour’.

A moderately active Member of the Cavalier Parliament, O’Neill was appointed to sixteen committees, including the committee of elections and privileges in two sessions. He was one of four Members sent on 19 Feb. to ask the King to send Sir Henry Vane and General John Lambert to London for trial, and ten days later he served on the delegation from the Commons to thank him for his gracious speech. On 10 Mar. he was named to the committee to confirm the charter given to the Royal Fishery Company, a matter of great concern to his constituency. With two other courtiers, William Legge and Robert Phelips, he planned to introduce a bill ‘tending to amendment of debauchery and loose living’. They petitioned for the office of farmers or accountants for regulating ale-houses; but the bill never reached the statute book.

Despite his many rewards O’Neill was dissatisfied with his position at Court.

He wrote to Ormonde on 11 Oct. 1662:

”I know that I shall be but a porter still unless I have it from your favour, which I doubt not to come by the earnest you give me. ... You know my melancholy humour makes me apprehend more than there is cause for”.

There was certainly no cause for his apprehension; in March 1663 he was granted the very lucrative place of postmaster-general, to which the Stanhope family had an hereditary claim. But he lived to enjoy it for little more than a year. He was noted as a court dependant in 1664, when he was among those to whom the petition from the loyal and indigent officers was committed. On his death on 24 Oct. 1664, he left everything to his wife and was buried in the church of St Nicholas at Boughton Malherbe, his wife's estate.

At his death the King of England wrote:

‘Poor O’Neill died this afternoon of an ulcer in his guts. He was as honest a man as ever lived. I am sure I have lost a very good servant by it.’ He was buried at Boughton Malherbe, the last of the family. Clarendon described him as ‘a great observer and discerner of men’s natures and humours’ and ‘very dexterous in compliance when he found it useful. ... Though his inclinations were naturally to ease and luxury, his industry was indefatigable when his honour required it, or his particular interest, which he was never without and to which he was very indulgent, made it necessary or convenient.’

But Samuel Pepys, who had cousins in Ireland, believed that the death of ‘the great Daniel O’Neill’ would be to the content of all the Protestant land-claimants in that kin. Regardless of Samual Pepys comments Daniel was yet another amazing O'Neill from an amazing Clan.