The Céide Fields, Belderrig, and Rathlackan are extensive early Neolithic field systems in County Mayo, Republic of Ireland. The Céide Fields are thought to be the earliest field systems in Europe, and as such they are listed as a potential World Heritage site. For this project, the buried soils of the 3 sites were analyzed in order to determine the nature and extent of the prehistoric land use within the field systems. The aims were twofold: to identify material added as fertilizer, and to determine whether the land was used for pasture or for arable agriculture. Soil phosphates and bile acids from the Neolithic soils indicate low levels of input of herbivore dung, and also some human fecal material in the Céide Fields. The results suggest that the soils may have been fertilized with animal manure.