After a brief military interlude with the Norman invasion of Ireland, Knowth fell into the hands of the monks at Mellifont abbey. It seems that the mound was then again used as a grange or farm. Stone walls were built on top of the mound, and stone buildings within the walls. After the dissolution of the monasteries, the site was used mainly for agricultural purposes until most of the site was purchased by the state in 1939.
The east-west orientation of the passages at Knowth suggests astronomical alignment with the equinoxes. The alignment at Knowth does not occur today. This is due to a number of factors. First of all, the passages were discovered by later settlers and were, to some extent, destroyed or incorporated into souterrains. In other words, the original entrances to the passages were distorted or destroyed, so it is difficult to establish if an alignment existed in the first place. Secondly, the recent excavations (1962 onwards) under Prof. George Eogan resulted in the erection of a concrete slab wall inside the mounds west entrance, restricting any investigation into the possible alignments. It seems likely that the passages were intended to align.
A brief excavation of the site was carried out in 1941 by Professor Macallister. However, major full-scale excavations began on the site in 1962 and were undertaken by Professor George Eogan of University College Dublin. When his excavations began, very little was known about the full extent of the site. The entrances to the western and eastern passages were discovered in 1967 and 1968 respectively, and, slowly, the layers of activity at the site of Knowth were uncovered. The excavation has produced numerous books and reports on the findings. The archaeological excavation of Knowth East ended any chance of research on alignments when Professor George Eogan erected a concrete wall across the east-passage entrance. The most extensive research on alignments and astronomy at Knowth was carried out by American-Irish researcher Martin Brennan.