The letters from each voyage undertaken by Irving highlight the familiar outline of the transatlantic slave trade. Common to each voyage was the purchase of slaves on the coast of West Africa, their re-sale in the Americas and the return journey to Liverpool with bills of exchange to be drawn against a British merchant house and/or a cargo from Africa and the Caribbean. Although these are well-studied characteristics of the slave trade, Irving's letters shed some light on the complexities of the trade and the variable elements within this deceptively simple pattern.

The African destinations mentioned by Irving in his letters included some of the most important trading locations for British ships in the late eighteenth century. All of the voyages in which he participated between 1783 and 1788 obtained slaves in the Bight of Biafra. This area supplied more than 326,000 slaves to the transatlantic trade between 1780 and 1800 and accounted for over one-fifth of all slave exports in the period. Over 85 per cent of the Africans exported from the Bight of Biafra between 1740 and 1807 were carried in British vessels and the region was of particular importance to Liverpool traders. Lovejoy and Richardson point out that ‘the Bight of Biafra was evidently the cornerstone of Liverpool's slaving activities from 1725 through to 1807’.

Bonny had emerged as the leading slave trading port in the Bight of Biafra by the 1730s, and in the closing decades of the eighteenth century it supplied over two-thirds of all slaves exported from the region. Bonny was the principal location for slave purchases during Irving's voyages on the Vulture in 1783, the Jane in 1784 and the Princess Royal in 1787. This is not surprising since Irving was employed by William Boats and John Dawson whose firms dominated British slave trading at Bonny in the late eighteenth century. The Gold Coast, a minor trading region for British ships in the late eighteenth century, was the principal area from which Captain Irving obtained slaves during a voyage in 1791. Although the area classified as West Central Africa was of growing importance for Liverpool ships in the late eighteenth century, none of the voyages in which Irving was involved apparently visited this part of the African coast.