Trench, the youngest of fifteen children was born near Ballybrittas, Queen’s County in 1808, he was named Richard. His younger brother who died in infancy was christened William. However, from a young age his name was changed to William Steuart. His father, Thomas, was the Church of Ireland Dean of Kildare, his mother was the daughter of Walter Weldon of Rahinderry, Co.Laois, who was MP for Athy. He grew up in the family home Glemalyre (Glenamalire) in Bellegrove, 3kms from Portarlington. At the age of 13 he was sent to be educated at the Royal School, Armagh, where he spent six years. He later studied Classics and Science in Trinity College Dublin. On leaving Trinity he began to study agriculture and land management, carrying out improvements on his brother’s estate in Kilmorony in Co. Laois, once home of the Weldon family. He then acquired land in Cardtown in the Slieve Bloom Mountains, where he was involved in carrying out reclamation, and began extensive potato cultivation.
In 1832 he married Elizabeth Sealy Townsend, whose father was the Master of Chancery in Ireland at the time. They had one daughter, Anna Maria, born 30 January 1836. Thomas Weldon Trench was born on 11 February 1833 and John Townsend Trench was born on 17 February 1834.(Elizabeth's brother, Richard Townsend , later married William Steuart’s sister Helena.)
Trench was an author. His most famous piece of work was 'Realities of Irish Life' which was published in London in 1868. He also wrote 'Ierne' (London, 1871), and produced many short articles including sketches published in the 'Evening Hours' (London, 1871-72). However, for most of his life he was employed as a professional land agent, a position he considered himself well qualified for. He claims in 'Realities' “to have lost no opportunity of acquiring information which might qualify me to become a land agent as being the most suitable, in its higher branches to my capacity.” He was appointed agent to the Shirley estate in Co Monaghan in April 1843 but resigned in April 1845. In December 1849 he was appointed agent to the estates of the Marquess of Lansdowne in Co Kerry. Two years later he took charge of the property of the Marquess of Bath in Co. Monaghan and that of Lord Digby in King's County in 1857 and held these appointments close to his death in 1872.
Trench was noted for the improvements carried out on the estates he managed. Under his agency the Barony of Geashill experienced a golden age of prosperity. There were vast improvements in both the architecture of the village and the topography of the landscape. Such improvements gained both national and indeed international recognition for Lord Digby and earned Trench the legacy of an improver who was well ahead of his time. Similar improvements in the layout and quality of the land as well as the construction of dwellings for example a terrace of cottages in Carrickmacross were built by Trench for the key workers on the Bath estate in Co. Monaghan. With the help of his son John Townsend Trench he transformed the Lansdowne estate by improving the dwelling houses of the tenantry, raising the standard of farming, improving local services and developing local industries and fishing. He had similar plans for the Shirley Estate, which he was not allowed to pursue, leading him to resign his agency.
To facilitate such improvements Trench embarked upon the implementation of assisted emigration schemes or what he referred to as ‘voluntary’ emigration. For example in assisted emigration from the Shirley estate 1843-54, it is noted that by 1851 the flow of assisted emigrants was reduced to a trickle. However, that year marked the beginning of a significant outflow of assisted emigrants from the neighbouring Bath estate, under the agency of William Steuart Trench. Similar schemes were carried out while he worked in Kenmare in Co Kerry, where he helped to ship over 4000 destitute people to the United States and Canada. Trench looked upon such schemes as a cheap and efficient way to improve the estate. While Trench was noted for his assisted emigration schemes in Counties Kerry and Monaghan, records show that the amount of money spent on emigration in Geashill was mimimal. Nonetheless the population decreased by 40.3%. during the Trench years suggesting that in an attempt to clear the estate to make way for improvements, many people were forced to leave, receiving little or no money from him.
While Trench’s physical legacy is a positive one, there is much evidence of discontent among the tenantry especially in counties Offaly and Monaghan, which led to a flame of agitation and the rise of Ribbonism. His physical legacy was greatly overshadowed by the realities of an agent who broke leases, levelled people’s homes and banished the poor and in so doing was often ruthless in his management of the estates in his charge. In a gradual process between, December 1871 and January 1872, Reginald Digby, Lord Digby’s nephew, replaced the Trenches as land agent in Geashill. Both Trench and his son Thomas Weldon died within a few days of each other in 1872 and were buried in Co. Monaghan in Donaghmoyne cemetery.
Many of Trench’s relatives were also employed as agents throughout the country: his cousin, Benjamin Bloomfield Trench worked as agent to Charles Verner (1854) and was also agent on the Bath estate 1868-1875. George Trench was employed on the Talbot-Crosbie estate in Co. Kerry while his cousin, William Trench acted as agent on the Heywood estate in Queen’s County. Other members of the Trench family held important positions in the church, for example: Richard Chenevix Trench was archbishop of Dublin from 1864 to 1884.