In March 2021 an effusive eruption began at the Fagradalsfjall volcanic system in Iceland, ending nearly 800 years of dormancy on the Reykjanes peninsula. The eruption produced lava flows and moderate gas emissions and, although it did not cause significant disruption, highlighted the need for near real-time monitoring of volcanic activity on the peninsula for future eruptions. The activity passed through several phases, each characterised by a different eruption style, providing a rich testbed for monitoring methodologies. We present measurements of the volcanic sulphur dioxide (SO2) emission rate and injection altitude throughout the eruption, generated by combining satellite SO2 imagery from TROPOMI with PlumeTraj, a back-trajectory analysis toolkit. We compare the results with ground-based measurements of the emission rate and plume altitude, finding excellent agreement in the plume altitude. Reasonable agreement was also found between the measured emission rates, with the best match for stronger and more continuous emissions. This demonstrates the ability for PlumeTraj to monitor SO2 emissions from future effusive eruptions, while highlighting the need for care when analysing results from low altitude plumes or during periods of high cloud cover.