The Irminger Current (IC) brings relatively warm and saline waters northward in the North Atlantic subpolar gyre, contributing to the upper limb of the Atlantic Meridional Overturning Circulation. The IC is a two-core current with surface-intensified velocities. The eastern core, closest to the Reykjanes Ridge, is warmer and more saline than the western core. To investigate the source waters of the two IC cores, using a 1/10° ocean model, we track Lagrangian particles released in the IC at OSNAP East (∼59.5°N) in the upper 1,000 m backward in time for one model year. Over a 1-year time scale, nearly all particles are sourced from nearby regions of the Irminger Sea and Iceland Basin. Those seeded in the western IC core mostly originate from the Irminger Sea (83%), while those in its eastern core mostly originate from the Iceland Basin (69%). Iceland Basin water feeding the IC predominantly crosses the Reykjanes Ridge near 57°N and 59°N. Generally, particles from the Irminger Sea are colder and fresher than particles from the Iceland Basin. The fraction of waters from the Iceland Basin and the Irminger Sea varies from month to month. So, to explain monthly variations of the two IC cores at the OSNAP East line, changes in hydrographic properties in both basins as well as their contributions must be considered. Based on this model study, we interpret the Irminger Sea circulation as a basin-wide recirculation with an increasing contribution of Iceland Basin waters toward the ridge which is subject to monthly variations.