Studied at University College Dublin and Trinity College Dublin. He was a research associate at Scoil an Léinn Cheiltigh, NUI Maynooth. Previously he taught early Irish history & settlement studies at Trinity, St. Patrick's College Drumcondra and NUI Maynooth where he lectured on the Medieval Irish Studies Programme at the Department of Old and Middle Irish. Prior to that he taught History and Irish in Belcamp College Secondary School. In recent years, he led and managed the Monasticon Hibernicum Project (funded by the Irish Research Council for the Humanities and Social Sciences). This Database of Early Christian Ecclesiastical Settlement in Ireland 5th to 12th centuries was published on-line in 2009. Has published a number of papers on early Irish political and ecclesiastical history and has contributed over 300 entries to the Dictionary of Irish Biography. Mac Shamhráin died in Dublin on 29 June 2011, after a very long illness which he endured with immense determination and dignity during which he continued to write and completed his final article in hospital on the date of his untimely death.
Prosopographica Glindelachensis: the Monastic Church of Glendalough and its Community, Sixth to Thirteenth Centuries, in Journal of the Royal Society of Antiquaries of Ireland, No. 119, pp. 82–84. 1989.
The Uí Muiredaig and the Abbey of Glendalough in the Eleventh to Thirteenth Centuries, in Cambrian Medieval Celtic Studies, No. 25, pp. 55–75. 1993.
Church and polity in pre-Norman Ireland, May 1996.
"Nebulae discutiuntur"? The emergence of Clann Cholmáin, sixth-eighth centuries, pp. 83–97, in Alfred P. Smyth (ed.), Seanchas: Studies in Early and Medieval Irish Archaeology, History and Literature in Honour of Francis J. Byrne. Dublin: Four Courts, 2000. ISBN 978-1-85182-489-2
The Vikings: an illustrated history, Dublin, 2002.
The Island of St Patrick: Church and ruling dynasties in Fingal and Meath, 400-1148, (ed.) Dublin: Four Courts, 2004
An ecclesiastical enclosure in the townland of Grange, parish of Holmpatrick, ibid, pp. 52–60
Church and dynasty in Early Christian Brega: Lusk, Inis Pátraic and the cast of Máel-Finnia, king and saint, ibid, pp. 125–39.
Database of Early Christian Ecclesiastical Settlement in Ireland 5th - 12th Centuries (with N. White & A. Breen) Dublin Institute of Advanced Studies (DIAS) 2009