The medieval Latin spellings of this name as recorded in the Papal Taxation of about 1302–6, are as follows: 'Haccudeo', 'Hacudeo'. We can see from these examples that the final part of the name was already pronounced as Deo by the beginning of the fourteenth century. Moving forward in time to the Ordnance Survey documentation of 1841, the name was written as Achadh d’eó in the Parish Namebook where it was attributed to a ‘local’ informant. In the same source Achadh deó is written in pencil, the usual method of recording names as pronounced in the locality. In other words, the placename was pronounced by local Irish speakers as Achadh Deo in 1841, as it had been, seemingly, from the beginning of the 14th century.
A letter by Thomas O’Connor of the Ordnance Survey dated 1841 states: ‘The name of this Parish is pronounced in Irish Achadh D’eó, which correctly written is, Achadh Da Eó … See Annals of the Four Masters and the Annals of Innisfallen …’. The 'Annals of Innisfallen' to which O’Connor refers here is in fact the source known as the Dublin Annals of Innisfallen, a late compilation of annals made in the year 1765. When we turn to the ‘original’ Annals of Innisfallen, partly written at Innisfallen, Co. Kerry, we discover that the placename is written as follows: abb Achid Déo (dated 939), i nAchud Deo (1010), i nAchud Déo (1044), daim liac Achad Deo (1061), go hAchad Da Eo (1177), tempoll mor Acha Deo (1282), de Acha Deo (1308), airchindach Acha Da Eo (1450). In general, therefore, the placename Aghadoe as recorded by the various Munster scribes who wrote the Annals of Innisfallen, had but one syllable in the final part of the name, i.e., Deó. As these annalistic entries are regarded as contemporary from at least the late 11th century, they provide us with clear evidence that the Irish form Achadh Deo has been in use for more than a thousand years. There are very few examples of Achadh Dá Eo (three words) in Irish historical texts, apart from the two references in the Annals Innisfallen already cited (dated 1177 and 1450 respectively), as well as Achadh Da Eo (and a variant spelling) recorded in Miscellaneous Irish Annals (1158) and one later entry in the well-known 'Annals of the Four Masters' (dated 1581).

A further example of Achadh Deo is found in an early 18th century Irish text known as as Pairlement Chloinne Tomáis which was probably composed by a Kerry author (see Dánta Aodhagáin Uí Rathaille). The Leabhar Breac manuscript notes to the Féilire or Martyrology of Óengus also mention ‘Achad Deo in iarthar Érenn’, which presumably refers to the same place.
‘An Seabhac’, Pádraig Ó Siochfhradha, while writing 'Achadh Dá Eó' on the index card for this name in 1955 also wrote: ‘Achadh D’Eo sa ghnáth-chainnt agus san sa Bhéarla féin’, which means 'Achadh D’Eo in ordinary speech and even in English’. Breandán Ó Cíobháin, another placenames scholar from Co. Kerry, favours the spelling Achadh Deo in his collection of minor names from this area (see Toponomia Hiberniae, volume I).
(P Ó Cearbhaill)