Ward sent a number of his colleagues, notably Michael O'Clery, to Ireland and elsewhere to collect or copy manuscripts, but died before much progress was made.

In 1645 he published at Louvain the Acta Sanctorum Hiberniae, containing the lives of the Irish saints whose feasts occur in the calendar for the months of January, February, and March.

Besides the "Lives" in the Trias Thaumaturga, there are also contained in this volume many valuable "Appendices", dealing with the ecclesiastical antiquities of Ireland, and critical and topographical notes, which, though not always correct, are of assistance to the student.

In 1655 he published at Antwerp a life of Duns Scotus, in which he undertook to prove that this great Franciscan doctor was born in Ireland, and not in England, as was then asserted.

The 1913 Catholic Encyclopedia praises Colgan's ability, industry, and critical sense; Seamus Deane calls him one of the more "scientific" historians of his time.