This thesis aims at an exploration of the ascetical theology and praxis of the sixth to the eighth century Irish monasticism viewed as a radical response to the Christian evangelium. It also aims to analyse the extent to which the distinctive response of the monastic Irish in the period arose from their Celtic cultural context. Culture influences all aspects of life and given that this work is addressing the critical period of the emergence of a people from primitive forms of religious belief and practice to Christianity it would be important to evaluate the influence of culture. The thesis is an exploration in the sense that, though much has been written about monasticism and specifically the Irish monastic movement up to and beyond the tenth century, the discussion of the ascetical theology and praxis has the potential to open up new pathways to better understanding and appreciation of this phenomenon within the wider Irish Church.  The scope of the work ranges briefly over the cultural context of Irish society in the pre and post-Christian era: its social organisation, sagas, Brehon laws and druidism. The primary sources utilised include the penitentials, the monastic rules, the Vitae and writings of ColumCille and Columbanus. These formative works regarding two of the most influential early Irish monastic founders are seen as encapsulating, and broadly illustrating, the ascetical emphasis and praxis of this time. The work draws on the ancient notions and practices of asceticism and the principle of contraries brought to light by Cassian. One facet of the radicality of Irish monasticism, manifested specifically in the penitentials, lies partly in the fact that, whereas asceticism is usually perceived as a personal response to the call to change one's lifestyle, the Irish praxis was, on the whole, undertaken in the context of a community.  Chapter One looks briefly at the Irish Church as part of that phenomenon called the Celtic Church. Other aspects of the topic addressed in this chapter include history (the Irish of the period had a particular way of looking at it), theology, asceticism, radicality and how each of these facilitates the future analysing of the primary sources. Chapter Two analyses the Irish penitentials that traditionally, and often today, have been seen as harsh and inflexible. Chapter Three analyses the monastic rules of some early founders and demonstrates that they are a call to a radical lifestyle for those committed to the religious life, compared with the ordinary demands of the Christian evangel. In Chapters Four and Five, the lives and writing of ColumCille and Columbanus are treated. The Sermons of Columbanus are the primary material used in Chapter Five. The conclusions of this work are that the radicality in the monastic rules, penitentials and the Vitae of its most prominent founders reveals that all the practices were designed to promote personal growth in the spiritual life and were not primarily focussed on punishment. They were about an inner transformation that enhanced one's personal, spiritual and human well being rather than a humiliation and belittling of the person. Present day psychology and the behavioural sciences in general would affirm the wisdom of the fundamental belief inherent in Cassian's contraries, which underpinned the injunctions in both the monastic rules and penitentials. The evidence deduced from many of the injunctions in the extant penitentials is that of a balanced presentation of the ideals of asceticism, which were a guide for the inner transformation of the person. Both the penitentials and the monastic rules also point to the emphasis on individuality that is evident in much Irish secular writing. The injunctions of the extant rules make it clear that their asceticism was, through prayer, sacrifices or mortification and work, to aid in the transforming of the energy of self-denial into a spiritual power. The asceticism thus recommended in these primary sources of the sixth to the eighth century Irish monastic movements was not harsh and inhuman, for the radicality of their lives depended on the fact that it was deliberately and personally chosen by the monks. They were captured by the beauty of their newly found faith in the Christian God, incarnated in Jesus whose life they contemplated in the daily recitation of the Canonical Hours and whose presence surrounded them in the totality of creation.