Notwithstanding its ambitious title, the substance of this book is a study of Irish-born Presbyterian ministers in what became the USA. The Canada region is excluded, and only one of the five chronological chapters is devoted to the nineteenth century (far more words concern the 1683–1810 period, the abundance of personal examples being inversely proportional to the number of “clerical migrants”). No attention is given to non-Presbyterian Protestant “dissenters” such as Methodists, Baptists, and Congregationalists. A large minority of Sherling's clerical migrants (almost a third of all those identified for the nineteenth century) were in fact Irish emigrants whose ministerial training was in America, a group that may have had little in common with the Irish-trained ministers who chose the evangelical pathway to America. No attempt has been made to construct a collective profile of these ministers from genealogical and census sources in both countries, so many of which are now digitally searchable. This book is nevertheless a valuable and solidly documented survey of over five hundred Irish-born ministers, classified (where known) according to place of origin, place of settlement, and date of migration.

Sherling laboriously chronicles important changes over time, such as the slow shift in Irish origins from northwest to northeast Ulster, and the persistent dominance of Pennsylvania despite increasing settlement in New York rather than the early concentration in Delaware, Virginia, and Maryland. Notable surges in clerical migration occurred around 1718, in the mid-1730s, and in the 1770s and 1790s as revolutionary turmoil engulfed first America and then Ireland. But the heaviest intakes (apart from 1718) occurred in the nineteenth century, at the end of the Napoleonic Wars, during the Great Famine and its immediate aftermath, and in other years according to a fuzzily cyclical pattern. It is a pity that most of the graphs and statistical tables relate to quite short periods, leaving it to readers to cobble together long-term trends without the aid of a graph covering the years from 1683 to 1901 or summary tables highlighting geographical changes. In this and other respects, Sherling's book should have been heavily edited before publication, in order to eliminate repetition, highlight major findings, and cut digressive footnotes.

At a higher level, The Invisible Irish is intended to develop the pioneering work of Sherling's mentor and thesis supervisor, Donald Akenson, by conducting a “thought experiment.” Akenson has repeatedly deployed data from Canada and other countries of Irish settlement to challenge easy assumptions about the character of emigration to the USA, especially its religious profile. In the absence of an American religious census, it has been all too easy for historians and the public to accept an Irish American stereotype embodying rural poverty, Catholic piety, and transplanted Anglophobia. This particularly applies to nineteenth-century mass migration, within which Protestant strands are far less visible and less studied those of the eighteenth century. The impediment to refuting this dubious stereotype is the unavoidable reliance on analogy for lack of direct evidence. Sherling's strategy is to point out similarities between clerical migration and Irish Presbyterian movement before 1810, and to put forward the hypothesis that the more obscure nineteenth-century movement may likewise have been mirrored in that of the 250 or so ministers who left Ireland for the USA in that period. If so, the time chart of Presbyterian movement would have resembled that of Irish emigration in general, as would the diversification in regions of origin and settlement. The hypothesis is plausible, and in due course the aggregation of genealogical data based on American church registers and headstone inscriptions may enable it to be tested. Meanwhile, The Invisible Irish is a useful contribution to Akenson's challenging agenda.

In one respect, Sherling has not fully implemented the sage advice of that “venerable historian” (!), who informed the “novice” that “context is crucial” (243). “Tunnel vision,” an optical condition widespread and often incurable, is evident in the absence of comparisons with other clerical migrations, and in the failure to analyse the relative influence of Irish and Scottish clergy and practices in American Presbyterianism. Wider reading would have enabled Sherling to relate his findings to the broader study of transnational religious institutions. No reference is made to Norman W. Taggart's The Irish in World Methodism, 1760–1900 (1986), David Hempton's many studies of international Methodism and evangelicalism, or recent articles by Andrew Holmes on Irish Presbyterian missionaries in the USA. Nor does Sherling explore the missionary dimension of Irish clerical migration, a major theme in Sarah Roddy's Population, Providence and Empire: The Churches and Emigration from Nineteenth-Century Ireland (2014). Even so, this book illuminates the traffic that passed through a previously murky historical tunnel, inviting others to recapture other strands of America's “invisible Irish” who failed to conform to the ethnic stereotype.