Casa Grande Ruins National Monument Boundary Modification Act of 2019 This bill authorizes the Department of the Interior to acquire specified private or state lands or interests by donation, exchange, or purchase from willing sellers in order to expand the boundaries of the Casa Grande Ruins National Monument in Arizona. Two parcels of specified federal land, one administered by the Bureau of Indian Affairs (BIA) and one by the Bureau of Land Management, are transferred to the National Park Service (NPS) for inclusion in the monument. Specified NPS land is transferred to the BIA to be used for the San Carlos Irrigation Project. Interior may enter into an agreement with Arizona to provide for cooperative management of approximately 200 acres of specified state trust lands.