In 1938 he spent a year cataloguing Irish manuscripts in the British Museum under the direction of the scholar Robin Flower. But he regarded exile in London as a dire punishment and he returned to Ireland in 1939 as Inspector for The Irish Manuscripts Commission – a body which oversaw the collecting of manuscripts kept in the Big Houses and which were a great source of social history and genealogy. He spent four years travelling the country in search of these manuscripts. At the end of his term in 1943 he had 87 collections edited and had 700 Irish manuscripts collected.

Fuair sé post sa British Museum faoin scoláire Robin Flower ag clárú lamhscribhinní ann. Bhí spéis mhór ag Éamonn san obair agus is uaidh seo a thosnaigh saothar mór a shaoil. Arais in Éirinn dó; ceapadh é ina Chigire taistil do Coimisiún na Lamhscribhinní i 1939, ag bailiú is ag clárú na sean cháipéisí ó na tithe móra. Sular éirigh sé as a ghnó cigireachta bhí 87 bailiúcháin de pháipéiri a bhain le saol na ndaoinei ins na tithe móra maraon le 700 lámhscribhinn Gaeilge a chur le heolas agus saibhreas na teanga.