Co. Londonderry was the plantation of the group of London guilds known as the Irish Society and was dominated by their tenants-in-chief who held large renewable leases at very advantageous rates. In 1785 Londonderry was described as: 'a Protestant County. In a well disposed state. Mr Conolly, Lord Tyrone (0113), Mr Cary, Mr Richardson,the Bishop of Derry and Mr Alexander (0029) have the chief influence. Mr Conolly has very great influence on this County and governs its sentiments in general.'247 Another interest was the Stewarts of Newtown(ards), who, like the Alexanders of Caledon, were originally Londonderry merchants. The Conolly leases were inherited from Thomas Conolly's great-uncle, Speaker William Conolly, along with the Manor of Limavady which William Conolly purchased in 1697 from the Phillips family. William Conolly had originally come from this part of the country and the principal Conolly estates were in Londonderry and Donegal, although their principal residence was their great house at Castletown in Co. Kildare.
The other great leaseholders were the Beresfords, who claimed part and finally all of the borough of Coleraine. The Marquess of Waterford leased 24,000 acres from the Drapers' Guild.248 The Beresfords lived partly in the county, as although the Marquess of Waterford's principal residence was at Curraghmore, Co. Waterford, his brother, John Beresford's (0115) estate was Walworth Manor, near Londonderry city, and at the end of the century his son, John Claudius, was agent for the Irish Society. The Carys held leases mainly under the Skinners' Guild and they were connencted by marriage to the Beresfords, as was Sir George FitzGerald Hill. The Rowley family is complicated, but through a marriage with a Dawson heiress it had a claim to a lease held under the Drapers' Guild.249 The bishopric of Derry was one of the richest in the Established Church and the bishop had large estates as well as an interest in the Bann fisheries. In the sixteenth century the Rowleys had had leases under the see of Derry, and they may have held them into the eighteenth century. Certainly there were two very formidable bishops during the century: Bishop William King, later Archbishop of Dublin, during the reign of Queen Anne, who carried on an internecine war with the Irish Society; and Bishop Hervey, later Earl of Bristol, who interested himself in the Volunteers and the Catholic question. Both bishops had a considerable interest in the temporalities of the church, though for very different reasons.
For most of the century the county was dominated by the Conollys and the Carys, whose principal residence was at Dungiven. In 1729 at the very brief (4-6 December) by-election following the death of Speaker Conolly, Arthur Dawson was returned by a majority of 19 votes over William Richardson (1785), the agent for the Irish Society; when Arthur Dawson became a Baron of the Exchequer, Edward Cary was elected in his place in January 1741/2. In April 1776 Lord Abercorn was told that 'It is believed that there will be a contest both in the city and county of Londonderry and generally thought that the late members for both will be re-elected.' By 2 June the election was over and Conolly had a 'very large' majority - 'very near 600'. John Richardson (1782) led Cary until the last two days, when Cary emerged with a considerable ('about 200') majority over him.
Thomas Conolly was returned for Co Londonderry for the last time in 1797: he was childless and thinking of retiring; also he had financial problems. In 1798 he sold his borough of Ballyshannon to Lord Belmore (1269). Six years earlier, in 1792, he had sold Newtown Limavady to Lord Londonderry (2008). Then in 1800 he resigned his seat for Co. Londonderry. At the ensuing election, Conolly supported Charles William Stewart, Lord Londonderry's second son, as his successor and despite threatened opposition from the Ponsonbys, Stewart was elected without opposition.250 Castlereagh (2009), who had married Conolly's niece, Lady Emily Hobart, was anxious to achieve this election as the MP would sit in the united parliament.
Londonderry's three boroughs - Coleraine, Londonderry city and Newtown Limavady - were all part of the plantation.