He served as MP in the House of Commons of the United Kingdom of Great Britain and Ireland and as member of the Irish Parliamentary Party represented Mid Tipperary from 1890 to 1892.

He later served as a Royal Irish Regiment officer with the New British Army in World War I, was an extensive writer, and proponent of improved relations between the United Kingdom and Ireland.

She was the daughter of Robert James Tennent, who had been Liberal MP for Belfast from 1847 to 1852 and a great-niece of the United Irishman Henry Joy McCracken hanged in 1798.

At this time, the Land War was in progress and in 1889 Harrison went to Ireland to visit the scene of the evictions in Gweedore, County Donegal.

He came to prominence briefly again in 1903 when, in spite of his lack of legal training, he successfully conducted his own case in a court action all the way to the House of Lords.

Later, Harrison successfully repulsed an attempt in the official history of The Times to rehabilitate that newspaper's role in using forged letters to attack Parnell in the later 1880s.

By the time of his death, he was the last survivor of the Irish Parliamentary Party led by Parnell, and as a member of the pre-1918 Irish Parliamentary Party, he seems to have been outlived only by John Patrick Hayden, who died a few months after him in 1954 and by Patrick Whitty and John Lymbrick Esmonde who were only MPs for a very short time during the First World War.