Jeffereyes (Jeffereys, Jeffries, Jeffreys), (James) St John (1734–80), MP and improving landlord, was born perhaps on the family estate at Blarney, Co. Cork, His grandfather, Brig.-gen. James Jeffreys (c.1650–1719/22), MP for Lismore (1703–15), married in succession two Swedish women; among the children was James St John's father, Capt. James Jeffreys, who was envoy and adviser to King Charles XII of Sweden from 1707, was taken prisoner at the battle of Poltava (1709), and was appointed (1718) British resident at the court of the tsar of Russia; his dispatches from Sweden and Moscow were of some importance. He died in 1739, survived by two daughters by his first wife and by his second or third wife Anne, daughter of St John Brodrick (qv), MP, who was mother of (James) St John Jeffereyes, and at least one other child. Relatives included prominent families in the north of Ireland as well as in Munster; John O'Neill (qv) (d. 1798), 1st Viscount O'Neill, was Anne Jeffereyes's nephew, and Trevor Hill, 1st Viscount Hillsborough, was her uncle.
St John Jeffereyes entered TCD on 12 February 1752, but seems not to have graduated, and entered the army. He attained the rank of major in the 24th Regiment of Foot (1766–7), and was lieutenant-governor of Cork 1768–9. He was able to combine his military activities with a parliamentary career; he was elected unopposed for Midleton, Co. Cork, in 1758, thanks to the Brodrick influence in the borough, and was reelected twice (1761, 1768). He married (5 June 1762) Arabella Fitzgbbon (qv) (as Arabella Jeffereyes), daughter of John Fitzgibbon (qv) (d. 1780) – a marriage that was to influence his career as much as his own family connections and local influence. In 1770 he became a trustee of the linen board for Munster, and from 1771 he held the post of commissioner of accounts at £500 a year. He was involved with three other landed gentlemen in establishing the Tonson Warren bank in Cork city (1768); they pledged £80,000 as security and £20,000 in working capital. It was a prominent institution in Cork until its failure in 1784, after Jeffereyes's death.
The bank's resources, as well as his linen board connection, were probably useful to Jeffereyes's ambitious improvements of his estate at Blarney. During the 1760s and 1770s he spent over £8,000 as loans to manufacturers or in building premises which he then leased out. Total investment in the ambitious schemes in the area was more than £20,000, and most of this was contributed or procured as grants and loans by the landlord himself. Blarney, 9 km from Cork city, was well situated and was well supplied with the water power crucial for industrial development; in 1764 Robert Stephenson (qv) described Jeffereyes's scheme for its improvement as ‘the most extensive plans for establishing a linen manufacture, and the most elegant plan of a village, that ever I did see in the British dominions’ (quoted in Dickson, 400). Thanks to Jeffereyes's policies of investment and of granting favourable leases, Blarney was transformed from two or three mud cabins in 1760 to a neat and prosperous village of ninety houses in 1776, when over 300 people were employed in linen manufacture alone, and with a church and inn built by the landlord. He realised the importance of good communications; he was able to organise the realignment of the road from Cork, and had bridges built. Jeffereyes's northern connections may have been significant; the large linen works at Bantry, founded in 1764 to employ eighty looms, was headed by John McCreight from Gilford, Co. Down, from 1769, and a number of Ulstermen were attracted to the area to work in other concerns. Jeffereyes encouraged textile printers to come from Dublin to set up in his village, in 1771–2, and a bleachworks was established in 1773; the thirteen factories in 1776 included a linen tape factory, a tuck mill, and a leather works.
In the 1770s he was an enthusiastic supporter of the Volunteer movement, like his cousin John O'Neill, 1st viscount O'Neill, who returned him to the family borough of Randalstown, Co. Antrim, in the election of 1776. Jeffereyes's limited political activity was mostly concerned with opposing his enemy Richard Boyle (qv), 2nd earl of Shannon, the local magnate. In 1778 Jeffereyes supported the popery bill granting catholics greater property rights; Shannon opposed it.
James Jeffereyes's name and the date 1759 appear on a stone in Rock Close, an unusual rock garden at Blarney, which was either built in that year as a folly, or was an existing ancient site which the young owner reconstructed to fit in with the taste of the day for ‘gothick’ or ‘druidical’ antiquities. Jeffereyes's early death, at his Dublin house on Hume St. on 14 September 1780, was a great blow to Blarney's development. His funeral was on 19 September in Blarney. He was survived by his widow and by a son and three daughters. One daughter married George Frederick Nugent (qv), earl of Westmeath, and another married Richard Butler (qv), 12th Baron Caher.