From the age of 17, Hart conducted a botanical survey of Donegal (lasting until 1898), which led to his publication Flora of the County Donegal, widely regarded as his most important botanical work. The publication was destroyed during a fire as part of the 1916 Easter Rising.[3]
In 1886, H. C. Hart wagered fifty guineas with the naturalist R. M. Barrington that he could walk the 111 km (69 mi) between the tram terminus in Terenure in Dublin, Ireland to the summit of Lugnaquilla in Wicklow and back in under 24 hours. Hart, accompanied by Sir Frederick Cullinan, walked from Terenure to Lugnaquilla, in the Wicklow Mountains - a total of 75 miles - and back in 24 hours, making a record at the time. Hart left Terenure at 10:58 pm on 20 June 1886 and arrived back at 10:48 pm the next evening.[4][5] The success of this challenge is remembered by the commemorative Hart Walk.[5]
In 1887, he married Edith Susan Anna Donelly (1852-1901), daughter of William Donnely, C.B., L.L.D, of Dublin. They had two daughters and divorced in 1897.[7]