I've read his book The Viceroys of Ireland and am trying to find out more about him. The book was published in 1912 and I can't find any more information about him.
I was able to dig up references to two people who might or might not be him:
A writer who published many detective books in the period 1925-1941.
A priest who helped De Valera escape from prison:
Collins and Boland went to London and then back to Dublin. The others journeyed back to Manchester by way of Sheffield. Milroy and McGarry were hidden by leading Manchester IRA commander Liam MacMahon in his own house, while De Valera stayed with a local priest, Father Charles O’Mahony. The police were looking for De Valera, of course, and MacMahon was warned by Thomas Walsh, a sympathetic detective in the Manchester force, that they were getting close. On 18th February, dressed as a priest and escorted by two young Irish women, De Valera travelled back to Dublin. At the beginning of June he went to the United States.
However, the first one seems to have little connection to Irish affairs, by the titles of his books, and the second one doesn't sound like the author of the book I've read (but who knows...).