The Inishtrahull Gneiss is considered to form part of the Rhinns complex that is also exposed on the islands of Islay and Colonsay.

[5] Among the algae present at Inishtrahull, Bonnemaisonia asparagoides, Callophyllis laciniata, Kalymenia reniformis, Sphaerococcus coronopifolius, Lomentaria articulate, L. orcadensis, Rhodymenia pseudopalmata, Plumaria plumose, Ceramium shuttleworthianum, Heterosiphonia plumose, Nitophyllum punctatum, Hypoglossum hypoglossoides, Drachiella spectabilis, Odonthalia dentate, Osmundea pinnatifida, Polysiphonia lanosa, P. stricta, Enteromorpha intestinalis, Ulva lactuca, Fucus serratus, F. spiralis and Ascophyllum nodosum have been recorded.

[6] The island has been designated an Important Bird Area (IBA) by BirdLife International because it supports breeding populations of several species of seabirds as well as being a winter feeding ground for barnacle geese.

The matter was raised in a Dáil debate on illegal fishing in November 1929 by the TD for the area, Deputy Frank Carney.

Responding, the Minister for Fisheries, Fionán Lynch TD said that he did not agree that the people had left the island because of illegal fishing and was making enquiries as to their current circumstances.

The Minister then reported in the Dáil that "four of the [six Inistrahull] families derived their livelihood from the Irish Lights boat service to the island, in which they are still able to engage" and that there was no case for state intervention.

There was, however, one milestone to pass, Inishtrahull lighthouse off the coast of Donegal was the last glimpse emigrants would have of Ireland [and] everyone stayed on deck until it disappeared.

The island and its adjacent 90-metre depth tidal sound attract basking sharks and cetaceans in large numbers during the summer months.