| "The advance on the north" : the difficulties of the Home Rule movement in south-east Ulster, 1870-1883 | Moran, Gerard | Article in Book | Queen's University Belfast | 1989 |
| A 1575 rent roll, with contemporaneous maps of the Bagenal estate in the Carlingford Lough district | O'Sullivan, Harold | Article in Journal | | 1987 |
| A history of local government in the county of Louth : from earliest times to the present time | O'Sullivan, Harold | Monograph or Collective Volume | Institute of Public Administration | 2000 |
| An arbitration award regarding intermixed lands in the parish of Dysart, dated 1671 | O'Sullivan, Harold | Article in Journal | | 1996 |
| Ardee dispensary minute book, 1813-1851 | O'Sullivan, Harold | Article in Journal | | 1965 |
| Carlingford | Gillespie, RaymondO'Sullivan, Harold, | Monograph or Collective Volume | Royal Irish Academy | 2011 |
| Copy of the bye-laws of the corporation of 'Atherdee' | O'Sullivan, Harold | Article in Journal | | 1965 |
| Dundalk | O'Sullivan, Harold | Monograph or Collective Volume | Royal Irish Academy | 2006 |
| Dundalk : a military history; with photographs and illustrations | Gavin, JosephO'Sullivan, Harold | Monograph or Collective Volume | Dundalgan Press | 1987 |
| Dundalk and north Louth : Cuchulainn's country | Clarke, GerryO'Sullivan, Harold | Monograph or Collective Volume | Cottage Publications | 1997 |
| Dundalk harbour improvements in the nineteenth century | O'Sullivan, Harold | Article in Journal | | 2002 |
| Dynamics of regional development : processes of assimilation and division in the marchland of south-east Ulster in late medieval and early modern Ireland | O'Sullivan, Harold | Article in Book | | 2005 |
| Introduction | Gillespie, RaymondO'Sullivan, Harold | Article in Book | | 1989 |
| John Bellew, a seventeenth century man of many parts, 1605-1679 | O'Sullivan, Harold | Monograph or Collective Volume | Irish Academic Press | 2000 |
| Land confiscations and plantations in County Armagh during the English Commonwealth and restoration periods, 1650 to 1680 | O'Sullivan, Harold | Article in Book | | 2001 |
| Landownership changes in the county of Louth in the seventeenth century | O'Sullivan, Harold | Thesis | | 1992 |
| Military operations in county Louth in the run-up to Cromwell's storming of Drogheda | O'Sullivan, Harold | Article in Journal | | 1990 |
| Rothe's Castle, Dundalk and Hugh O'Neill : a sixteenth century map | O'Sullivan, Harold | Article in Journal | | 1965 |
| The background to and the events of the insurrection of 1798 in Dundalk and North Louth | O'Sullivan, Harold | Article in Journal | | 1998 |
| The beginnings of the Catholic parishes of Dundalk, Haggardstown and Kilkerly in the county of Louth | O'Sullivan, Harold | Article in Journal | | 2003 |
| The borderlands : essays on the history of the Ulster-Leinster border | | Monograph or Collective Volume | Queen's University of Belfast | 1989 |
| The Catholic parishes in the Barony of Cooley | O'Sullivan, Harold | Article in Journal | | 2015 |
| The county of Louth | O'Sullivan, Harold | Article in Journal | | 1984 |
| The courthouse, Dundalk, and the contract for its erection dated 30th April 1813 | O'Sullivan, Harold | Article in Journal | | 1962 |
| The Cromwellian and Restoration settlements in the civil parish of Dundalk, 1649 to 1673 | O'Sullivan, Harold | Article in Journal | | 1978 |
| The eighteenth-century maps of the Clanbrassil estate, Dundalk | O'Sullivan, Harold | Article in Journal | | 1961 |
| The emergence of the national system of education in Co. Louth | O'Sullivan, Harold | Article in Journal | | 1973 |
| The Franciscan friary at Shean | O'Sullivan, Harold | Article in Journal | | 1972 |
| The Franciscans in Dundalk | O'Sullivan, Harold | Article in Journal | | 1961 |
| The grant of arms to the Louth county council | O'Sullivan, Harold | Article in Journal | | 1978 |
| The house on the ridge of the weir : the story of the brothers and community of Saint John of God, Saint Mary's Drumcar : fifty years serving the needs of persons with learning disabilities in the county of Louth | O'Sullivan, Harold | Monograph or Collective Volume | Drumcar Park Enterprises | 1998 |
| The Jacobite ascendancy and Williamite revolution and confiscations in County Louth, 1684-1701 | O'Sullivan, Harold | Article in Journal | | 1992 |
| The landed gentry of the county of Louth in the age of the Tudors | O'Sullivan, Harold | Article in Journal | | 1989 |
| The Magennis lordship of Iveagh in the early modern period, 1534 to 1691 | O'Sullivan, Harold | Article in Book | | 1997 |
| The march of south-east Ulster in the fifteenth and sixteenth centuries : a period of change | O'Sullivan, Harold | Article in Book | | 1989 |
| The plantation of the Cromwellian soldiers in the barony of Arda, 1652-1656 | O'Sullivan, Harold | Article in Journal | | 1988 |
| The restoration land settlement in the diocese of Armagh, 1660 to 1684 | O'Sullivan, Harold | Article in Journal | | 1994 |
| The Trevors of Rostrevor: a British colonial family in seventeenth-century Ireland | O'Sullivan, Harold | Thesis | | 1985 |
| TheTichborne acquisition of the Plunkett estate of Beaulie | O'Sullivan, Harold | Article in Journal | | 1990 |
| Two eighteenth century maps of the Clanbrasil estate, Dundalk | O'Sullivan, Harold | Article in Journal | | 1961 |
| Women in county Louth in the seventeenth century | O'Sullivan, Harold | Article in Journal | | 1995 |