Rev. John Mackay was Free Church minister in St Kilda at the time of the visit by Lord Napier. He had held this charge for 18 years by that time, and we see him in the 1881 census. At that time, he is aged 64 and is quoted at the St Kilda manse with housekeeper Ann Macdonald, 9-year old scholar John Macdonald (boarder) and 67-year old Ann Mackinlay, nurse and teacher. She hailed from Bracadale in Skye.
The entry for the minister in the 1871 census is not easily found, but is soon spotted among the 87 people present on the island of St Kilda. John (54) is joined at the FC Manse by his sister Marge (40) and Ann Macdonald, housekeeper. Rev Mackay, who had been in office in St Kilda since 1844, does not show up on the 1851 or 1861 censuses. St Kilda cannot be differentiated in the 1841 census.
By 1891, the Rev Angus Viddes (48) is on the census as United Free Church minister for St Kilda, in the company of Catherine Maclennan (59), domestic servant, born in the district of Carloway in Lewis and a Jessie 'Chrouabl', a nurse. This minister, his surname transmuted to Fiddes, remains on St Kilda at the time of the 1901 census. He is quoted as being born at Tarbat in Rossshire.
The church history for St Kilda is adequately summarised on this Wikipedia page.