He was awarded his second doctorate degree (German: Habilitation) in 1985 at the University of Bonn where he was appointed professor of English linguistics in 1987.
His book publications have been and continue with major publishing houses such as Cambridge University Press, Wiley-Blackwell (Hoboken, New Jersey), Mouton de Gruyter (Berlin) and John Benjamins (Amsterdam).
[4] In 2020 he was appointed adjunct professor at the Faculty of Arts, Humanities and Social Sciences of the University of Limerick, Ireland.
This comprehensive work in six volumes is intended to reflect recent research insights as well as new theories and methods in English historical linguistics.
In the area of Irish phonology, Hickey has devised a maximally concise system of description which captures many linguistically valid generalisations about the sound structure of that language.