It is claimed that Eochaid became blind but that his sight was later miraculously restored. Thereafter, he was known by the sobriquet Dallán (little blind one). The implication, perhaps, is that he closed his eyes to the ways of the world. The additional sobriquet Forgaill, also given as mac Forgaill, is linked by many scholars with the verb for-gella (testifies or gives superior witness). While there is no certainty, it could reasonably be inferred that Dallán's family was of the grade aire forgill, a noble class of landed proprietor next in rank to royalty. There seems to be little basis, however, for the later tradition that his mother was named Forchella.
As poet, Dallán is credited with authorship of an ode to the Cenél Conaill dynast Áed son of Duach (d. 580) and with three eulogies in praise of distinguished saints. One of these eulogies, which apparently celebrated Conall of Inis Caíl (Iniskeel, Co. Donegal), no longer survives. The amra in praise of Senán (qv) of Inis Cathaig (Scattery Island, Co. Clare) resembles the better known amra dedicated to Colum Cille (qv) of Iona, the preface of which ascribes it to Dallán. He was present, it is claimed, at the synod of Druim Cett (572/90) and composed a praise poem for Colum Cille in gratitude for his defence of the filid (poets). As Colum Cille had been cautioned not to give way to vanity, he dissuaded Dallán from reading the poem. On Colum Cille's death in 597, however, Dallán was moved to compose a eulogistic lament.
Dallán's role as church founder is reflected in version II of the above-mentioned Irish Life of Máedóc, where he is associated with Cell Dalláin (Kildallon, near Drumlane, Co. Cavan). Later sources link him with the church of Maigen (perhaps Moyne, Co. Mayo), while he may be commemorated by dedications at Dísert Dalláin (seemingly in Co. Cavan), Tulach Dalláin (Co. Donegal), and Cluain Dalláin, also in Ulster. Whether these represent any personal connection, or cult diffusion, is unclear. The foundations attributed to him do not feature in the subsequent historical record, but there was a hospital at Kildallon in the late medieval period. According to the late Acta SS Hib., Dallán suffered a martyr's death, being slain by pirates. A strong tradition maintains that he is buried at Conall's foundation of Inis Caíl. The martyrologies commemorate him at 29 January.