Originally a light was established on Inishmore near Dun Oghil at a height of 400 feet along side the Signal Tower. The light was first lit on 1 May 1818 with a revolving optic giving it a flashing character so as not to be confused with either Loophead to the south or Clare Island to the north, both of which were fixed or non-flashing lights.
The optic had three faces each with seven oil lamps and catoptic reflectors which when revolved gave a 30 second flash every three minutes. The Revenue Commissioners approached the Ballast Board two or three times over the following years without avail requesting that the Inishmore lighthouse be repositioned as it was too high and did not cover the entrances to the North or South Sounds. Eventually a request from the Galway Harbour Commissioners in 1850 for an additional lighthouse on Aran North or Eeragh received favourable attention. Inspector George Halpin reported that lights on the extreme islands would have an important advantage. The Inspecting Committee agreed with Mr Halpin and ordered that light on Eeragh was to be the same as Inishmore and Inisheer was to be a fixed light.
Trinity House gave their approval and said that if preference was to be given Inisheer should be constructed before Eeragh but if the Corporation decided to build both towers at the same time they, Trinity House, would not withhold their sanction.
Land was acquired and by 1853 Inspector Halpin reported that both towers had reached the second floor level, and dwellings were at the fourth course.
Chance Brothers of Birmingham supplied the fixed optic for Inisheer and Wilkins of London the flashing optic for Eeragh.
Due to weather conditions the exhibition of the lights was delayed from 1 November to the 1 December 1857, and at the same time Inishmore light was discontinued. Inisheer's first order fixed white light had a red sector over the Finnis rock.
Both towers and dwellings were built of the local very hard chrystaline limestone, subsequently the towers received distinguishing coloured bands.
The Inspecting Committee recommended, in October 1909, that the improvement to Inisheer's light should be included in the Estimates for 1910-1911. The light was to be converted to incandescent with a paraffin vapour burner and the character changed from fixed to occulting, now called isophase, 10 seconds light, 10 seconds dark, the conversion took place on 31st January 1913.
In the Development Committee's Forward Policy Paper of October 1973 they proposed that Inisheer should be automated and modernised in the year starting 1977/78 and placed in charge of an attendant who would be resident on the island.
By 31 March 1978 Inisheer was converted to unwatched electric with, three diesel generator sets, a mercury vapour lamp as the light source in the 1913 optic driven by small electric motors. A standby lantern on the main lantern balcony operates from a battery if there is a complete electric breakdown. There is a monitoring radio link between the station and the attendant's house a distance of 2 km (1.2 miles).