In Cologne sa Ghearmáin a rugadh an scoláire Ceiltise seo. Bhí sé ina leabharlannaí in Ollscoil Frieburg. Deir Seán Ó Lúing (Kuno Meyer, 1858-1919: a biography, 1991): ‘Hessen was a mere youngster, just twenty-four, awfully nice and full of the soundest knowledge but Meyer[B2] learned that he would only work when told or forced to and had deliberately chosen a librarian’s career in preference to an academic one . . . . Hessen’s plan was to collect into a single volume all the material hitherto scattered over dictionaries, vocabularies to particular texts, commentaries and older Irish glossaries. Sadly it was his fate to be killed in action in the war shortly to overwhelm Europe with his task only part-accomplished.’ I litir chuig Richard Irvine Best[B2] dar dáta 20 Meán Fómhair 1913 deir Meyer: ‘I shall now devote myself entirely to making Hessen’s work as thorough as possible: he is a candida anima, and if he has not the genius of C.M.[Carl Marstrander (B4)] his character and temper are superior to those of our Norsemen.’ Foilsíodh a thráchtas Zu den Umfarbungen der Vokale im Altirischen i 1912 agus tá a shaothar i gcló in Zeitschrift für celtische Philologie, in Indogermanische Forschungen agus in Zeitschrift für vergleichende Sprachforschung. Liostáil sé in arm na Gearmáine agus maraíodh sa chogadh é 29 Meán Fómhair 1915. Foilsíodh an chéad imleabhar de Hessen’s Irish Lexicon. A concise dictionary of Early Irish with definitions in German and English by Séamus Caomhánach[B3], Rudolf Hertz, Vernam E. Hull[B5] and Gustav Lemancher, S.J. with the assistance of many collaborators, 1933 agus an dara himleabhar i 1938. Dúradh sa réamhrá nach raibh ach beagán den obair réitithe ag Hessen féin sular cailleadh é.
Scríobh Gearóid Ó Murchadha[B2] léirmheasanna in Béaloideas, Iml. iv, Uimh. iii (1934) agus san iris chéanna sin i 1939. Deir sé: ‘All serious students of Irish will welcome this, the first fasciculus of what is bound to remain for long the standard students’ dictionary of Old, Middle and Early Modern Irish . . . . Both editors and publishers of Hessen’s Irish Lexicon deserve the warm thanks of all who are interested in Irish studies for the effort they are making to supply the student and general reader of Old and Middle Irish literature with an adequate dictionary. It may be some requital to them for the labour they have expended upon the Lexicon to know that, if the first fasciculus is a fair specimen of what is to follow, the completed work will indeed be a highly satisfactory work for the student and general reader, and will moreover be of considerable help to the research worker.’ I 1939 dúirt sé: ‘But what is remarkable about every fascicle of Hessen’s Lexicon, including the fascicle under review, is the fullness, concision and accuracy of almost all its articles.’ Níor foilsíodh ach an dá imleabhar.