Rennie was no stranger to Dublin, or indeed to Ireland. A small notebook in the National Library of Scotland records his 1801 impressions of the Dublin harbour area. Three years later, the army commander, Lord Cathcart, showed him the sites that had been selected for Martello towers in the Dunleary area. His Irish projects included acting as consulting engineer to the Royal Canal and Howth harbour projects, and a great ‘tobacco stores’ building at the Custom House docks in Dublin. In November 1814 stormy weather limited his review of potential harbour sites, but what he saw confirmed his earlier opinion, reported in April 1802, that ‘under all the circumstances . . . Dunleary, or rather a little to the east of it’, was the ‘most fit place’ for the asylum harbour. Developments now came quickly, with commissioners being appointed by the government to advance the building of the new harbour and Rennie being engaged as the directing engineer.
Assisted during 1815–16 by the surveyor Netlam Giles, Rennie made plans for the transport of the Dalkey granite, using three ‘inclined plains’ to bring it downslope from the quarry areas and then sending it by horse-drawn wagon along a specially constructed rail line to Dunleary. Simultaneously he prepared estimates for the cost of the single east pier that was the initial target. By early 1816 the plans were sufficiently advanced for 66 landowners to be advised of the intended rail line and for a petition to parliament for the required enabling act. The very precise estimate sent to parliament by Rennie foresaw the new pier as costing, over a ten-year period, £595,193, of which £84,198 19s. was for the iron railway from Dalkey. An accompanying plan shows that the pier was to be 2,800ft (c. 800m) long, with a 200ft-wide base. Interestingly, the pier then envisaged was to project from near the ‘Churl Rocks’ (approximately in line with the present car ferry jetty)—about 300m west of the location (already identified by Toutcher in 1811) where it was finall