He was named after Thomas MacDonagh one of the signatories of the Proclamation of the Irish Republic, who had been executed after the Easter Rising earlier that year.

His stance on standard forms and spellings was supported by Éamon de Valera despite opposition from traditionalists in the Department of Education, and the work is widely seen as an important benchmark in Irish scholarship.

Much of the material in this archive was later used as the basis of Niall Ó Dónaill's Foclóir Gaeilge-Béarla, published in 1978, for which he was consulting editor.

Also during the 1970s, de Bhaldraithe translated the Irish language diary of Amhlaoibh Ó Súilleabháin into English.

In later years he worked extensively on the definitive Irish dictionary, Foclóir na Nua-Ghaeilge, which remained unfinished when he died in 1996, but which is still in progress today.