The solution to the cryptogram, achieved by the substitution of Greek letters for Latin in accordance with a fixed table of correspondences, is found in a Brussels manuscript (Bibl. Roy. MS 9565–6). It begins: ‘Mermin rex Conchen salutem’ (Mermin [Merfyn], the king, greets Conchen [Conchenn]). The authors of the covering letter add a jibe at Dubthach's ignorance of contemporary British phonology in spelling Conchenn, with a single ‘n’ at the end. The identification of the Colgu in the covering letter with the Colgu (qv) (d. 789) grandson of Duinechaid who corresponded with Alcuin is not possible. Another cryptogram, involving the use of the same solution key, is found in a ninth-century Welsh manuscript (Cambridge University Library, MS 1285, p. 67, formerly Ff. 4. 42).
Dubthach is the scribe of most of Leyden, Universiteitsbibl. BPL 67, a copy of Priscian's ‘Institutiones grammaticae’, with other grammatical and glossary material, written in a variety of insular hands, one of which has been identified as the autograph of John Scottus Eriugena (qv). The computistical verses that Dubthach added on f. 7v date the manuscript precisely to 838. His name also appears among the glosses and marginalia of several Sedulian manuscripts, including Codex Sangallensis 48, a bilingual Greek-Latin copy of the gospels, the codex Boernerianus (now Dresden, Bibl. Misc. 145b), which contains a Greek-Latin copy of the Pauline epistles, and Codex Bernensis, a collection of classical Latin and other material. The full extent of Dubthach's undoubted knowledge of Greek needs to be tested palaeographically through a comparison of the hands in these manuscripts with the main hand in the Leyden Priscian.
The Annals of Ulster note his death in terms befitting Dubthach's exalted self-estimation: ‘Dubthach mac Máele Tuile doctissimus Latinorum totius Europae in Christo dormivit’ (Dubthach, the most learned Latinist of all Europe, died in Christ).