This article argues that the massive increase in transatlantic British and Irish emigration after 1840 was enabled by declining fares and ocean travel costs. New series of transatlantic steerage fares drawn from the unique Cope Line records at the Historical Society of Pennsylvania (HSP) show westward fares fell rapidly from 1830. Adjusted for British and US manual wages, westward travel costs, including provisions, almost halved between 1847 and 1851–3, when Irish migration peaked. Hence although the Irish had to leave Ireland, they might not otherwise have gone so extensively to North America. Eastward travel costs also fell after 1830, encouraging an unexpectedly large return migration to Britain in the late1850s, and maybe earlier.