Palaeodrainage models for the mixed fluvial-aeolian systems, which supplied detritus to Triassic basins on and offshore Britain and Ireland are well established. Basins such as those across Northern Ireland are not as well understood. Provenance studies of Triassic sandstones in the Slyne Basin offshore western Ireland and in basins west of Shetland have indicated that sediment supply was through a southward flowing fluvial system. Similar work on Triassic sandstones in the Wessex and East Irish Sea basins on and offshore Britain identified source rocks to the south supporting models, which evoke the northward flowing "Budleighensis" river system. The basins across Northern Ireland are potentially situated along the drainage divide between these two large-scale drainage systems. K-feldspar Pb-isotopic analysis, apatite U-Pb geochronology and trace element geochemistry identify the Hebridean Platform, and the Scottish and Irish massifs to the north and west, respectively, and the remnant Variscan Uplands to the far south of the basins as source areas. The proportion of the northern- and southern-derived detritus fluctuates several times over the sampled intervals, suggesting the dominance of drainage systems supplying sand to the basins "switched" intermittently over time. This may be due to abnormally heavy rains periodically powering the Budleighensis river system farther north or perhaps localised subsidence temporarily disconnecting Triassic basins on and offshore Britain and Ireland. The Triassic basins in Northern Ireland acted as either a major drainage divide between southern and northern river systems or as a regional sink for sediment preventing further expansion of either system.