Chevalier William Henry Grattan Flood (baptised 1 November 1857[1] – 6 August 1928) was a noted Irish author, composer, musicologist and historian.

He was born to William and Catherine (FitzSimon) Flood, the Master and Matron of the Lismore Union Workhouse.

He quickly became an accomplished pianist and, at the age of nine, was invited to give a recital for the Duke of Devonshire at Lismore Castle.

During his long residency at Enniscorthy (1895–1928) Flood authored the majority of his musical compositions and historical publications.

He has undoubtedly inspired a lot of more recent research, but "his appreciation of detail was enthusiastic rather than thorough, and the contents of his books were often distorted by his national and religious commitment".

"[9] Flood's most adventurous claims included an "Irish Ancestry of Garland, Dowland, Campion and Purcell".