Niall was a successful jockey and won his first race at the age of 16 in his native Ireland. He soon discovered his special talents as a trainer and traveled around the world working at tracks and farms in New Zealand, Australia, California and Kentucky, broadening his knowledge of the thoroughbred industry. In 1987, he relocated to Florida to train for George Steinbrenner at his Kinsman farm.
In 1991 Niall Brennan presented his first consignment at Select Two-Year-Old sales and since then has consistently ranked as a leading consignor in North America numerous times.
Niall is also an active member on the board of The Breeder's Cup, Ocala Breeder's Sales Company, Fasig Tipton Sales Company and is the president of the Ocala Farm Ministry.