The main question and arguments of this essay are encapsulated in its title. It examines how geographical (‘Hibernian’) and scientific (‘mainstream’, ‘tributary’) terminologies and systems of classification need to be historicized in a specific period (mid-nineteenth century) on two important accounts. First, the frames of reference for such terminologies and classifications may be different from today. Second, these frames of reference also shaped wider intercultural interactions and exchange. In using seemingly ahistorical river terms and metaphors and classical Latin/‘poetic’ geographies, the title thus draws attention to the need for more careful contextualization and questioning of assumptions about ‘Ireland’. For example, the development of geology, hydrogeology, and ichthyology as major disciplines and new sub-disciplines in nineteenth-century scientific endeavour brought revised scientific terminologies for river systems. An indicative contemporary definition below thus frames this essay, to locate its wider ramifications. In offering the first evaluation of the contributions of William Thompson's The Natural History of Ireland (1849–56) in the history of nineteenth-century ichthyology, this study therefore also re-evaluates the status of ‘Ireland’ in the history of nineteenth-century natural history. By arguing that both Thompson (1805–52) and (nineteenth-century) Ireland merit a more centrally contributory rather than auxiliary positioning in the history of scientific endeavour, the essay challenges the use by historians of allegedly a-temporal river metaphors: they are never culturally or historically neutral. In showing how Thompson's work occupies a more central place on the nineteenth-century scientific map as an important, comparative case study, the essay can then conclude with the ‘modest proposal’ that overtly territorialized and overly terrestrial conceptions of natural history endeavour lose sight of more significantly fluid and inter-connective scientific and cultural understandings of things.

According to Thompson's contemporary, the differently overlooked popularizer of science, Rosina M. Zornlin: