What is "biruaslum"

Biruaslum or Bioruaslam is a stack in the Barra Isles of Scotland, to the west of Vatersay. Approximately from the nearest road, it reaches in height and there is a ruined prehistoric fort on the southern side. Francis G. Thompson characterizes it as "high and virtually inaccessible"; James Fisher mentions a " fulmar flying up and down its tiny cliff."