Trim was first enfranchised by a charter of Walter de Lacy, Lord of Meath. By a charter of 1572, 13 Eliz. I, the corporation comprised a portreeve, burgesses and freemen unrestricted as to numbers. In 1783 Trim was a town of '2,000 inhabitants. Electors, a Sovereign, 12 Burgesses and about 300 Freemen under the direct influence of [its] Patron Lord, Mornington.'313 In 1790 it was said that:
This County assize town, is one of the most ancient Boroughs of Ireland. It was first the baronial seat of the Lacy family, afterwards of the Barnewalls. Parliaments have been holden there and Kings entertained in it with splendour.
The decay of this ancient Borough is however at this day too evident. The family of the great Duke of Wharton had the chief influence in this Borough from the Restoration to the Revolution. At this period, the Wharton property was sold for a mere trifle to Mr Garret (Wesley, 2211) the ancestor of the Mornington family hence their influence commenced, but the family of Ashe of Ashefield (now Sir Thomas Ashe's) ousted them and kept possession of the Borough for forty years. However, the fate of the Borough, now thirty years quietly enjoyed by Lord Mornington, is shortly to be decided in the King's Bench.314
The Belfast Newsletter reported the case, 'which involved the following question; Whether the Borough of Trim should be free or not'. The contest had gone on for four years, and:
during this period two general verdicts had been obtained against his Lordship at the Assizes of Trim, by juries composed of gentlemen of the first character and consequence in the county. The Court, however, thought proper to lay both aside, and contrary to the hopes and expectations of the public, pronounced against the plaintiffs. By this decision the new Burgesses, consisting of near three hundred, though declared by two juries 'agreeable to the strict letter and spirit of the charter and to the immemorial usage and custom of the corporation', are by this determination, stripped of that privilege.315
Lord Mornington established his control over the borough, which he used to start his distinguished brothers on their political careers. His next brother, William (2216), was returned in 1783, in 1790 he returned Arthur (2210), later 1st Duke of Wellington, while Henry (2213) was returned for a few months in 1795. As Marquess Wellesley, he was paid the £15,000 compensation for its disfranchisement at the Union.