- Medieval Ireland
- Sixteenth- and seventeenth-century Ireland, especially the Gaelic lordships of Ulster and especially the lordship of Tír Chonaill and the County of Donegal
- Irish noble and military exiles in Spain, Rome and Spanish Flanders in the early seventeenth century
Red Hugh O’Donnell and the Nine Years War (Dublin: Four Courts Press, 2005)
The Donegal Plantation and the Tír Chonaill Irish 1610-1710. Maynooth Local Studies Series (Dublin: Four Courts Press, 2010)
The Battle of Clontarf, Good Friday 1014 (Dublin: Four Courts Press, 2013)
Several articles published in books and journals mostly dealing with the sixteenth and early seventeenth century history of Co. Donegal.
Contributor of biographies on early modern people in the Dictionary of Irish Biography (Cambridge University Press, 2009), including Gaelic chieftains, bardic poets, a number of ecclesiastics, a famous doctor and some leading citizens of Galway.