One of the major issues facing the historian using the Irish chronicles is whether the abundant material found only in the Clonmacnoise group is valid as evidence for the period A.D. 431–730. An important first step in addressing this is to establish how this section of the chronicle developed in the Clonmacnoise-group texts after the common source ended in 911. The material for the period ca 490–766 found in AT, the most substantial Clonmacnoise-group source, but not in AU, has already been presented and discussed by David Dumville, who outlined many of the characteristics of this corpus of items. The focus in this chapter will be on aspects of secular kingships, but hopefully, in the future, other significant elements of the Clonmacnoise-group texts, for instance the abundant unique ecclesiastical material, will receive the attention they deserve. The intention of this chapter is to analyse in more detail how secular history was altered in the Clonmacnoise group, to gain some understanding of the view of the past portrayed in the chronicle and to assist scholars in the evaluation of the reliability of text unique to the Clonmacnoise group.