The fourth Irish History of Mathematics conference (IHoM4) was jointly organized by the Edward Worth Library (EWL, 1733) and the School of STEM Education, Innovation and Global Studies, in Dr Steevens’ Hospital (where EWL is located) on Friday 9th June. Twelve presentations were made to 37 enthusiastic participants. The presentations covered a period of a millennium and a half, from early medieval knotwork to Einstein’s cosmological constant. There was a focus on the lives of mathematicians, Robert Steell, James Thomson (father of Lord Kelvin), William Rowan Hamilton, Henry Smith and on the replication of Foucault’s pendulum experiment in Dublin in 1851. Early understandings of the limit concept were discussed, as was the use of Euler’s mathematics on students’ learning today. The other talks concerned mathematics in libraries: those founded by Narcissus Marsh and Edward Worth, and a virtual database of 750 ‘Irish’ books on mathematics.