This book is the first in-depth investigation of the expression of agency in verbal noun and impersonal/passive constructions in medieval Welsh and Irish, drawing on a database of texts from different genres: narrative, legal, and annalistic. The analysis is primarily data-oriented, rather than theory-oriented, although it draws on methods and concepts from functional grammar approaches and cognitive linguistics. Written with readers from a wide variety of backgrounds in mind, Agents in Early Welsh and Early Irish goes beyond earlier contributions in the field of Celtic syntax and semantics in several important ways: It presents data from both Welsh and Irish It discusses both the internal structure of verbal noun and passive/impersonal constructions and their contributions to their contexts It offers new analyses of agency and its markers in medieval Welsh and Irish, as well as analyses of the marking of instrument, cause, and indirect agency