How do you research an unrecorded dialect?
Quite simply: with great difficulty.
If you wish to record features of a dialect, in the simplest sense you find a number of people who speak that variant and begin recording how they speak it. Historically, this was quite a feat: speakers of the dialects that most interested researchers usually lived in the most remote and difficult to reach places, and travel was by foot. Upon locating people willing to be recorded, the researcher - or a put-upon assistant! - would then write down how those people spoke using only pen and paper, usually utilising their own phonetic shorthand, frantically copying down extensive passages for hours and days on end.
Finnish folklore collector Elias Lönnrot, 1828. Early dialect researchers were folklorists. (Source: Projekti Lönnrot.)
Technological advances during the late nineteenth and twentieth centuries vastly improved the situation. Roads were improved, bicycles, cars and public transport made journeys to even the most difficult to reach areas quicker, easier and more frequent, making it easier to find dialect speakers. Most importantly, the introduction of (at first rudimentary) recording devices revolutionised dialect study, allowing the sounds of a dialect to be captured and played back for the first time (although it was still sometimes the case that speakers themselves would have to travel to universities to be recorded!).
Wax cylinder recordings, 1897-1948. At right, Tadhg Ó Murchú recording from Pádraig Ó Súilleabháin (Source: UCD National Folklore Collection)
The introduction of a common phonetic alphabet also facilitated the easier transcription and comparison of features between linguists. Finally, the internet has revolutionised the field once again, providing opportunities for researchers and dialect speakers to contact one another, information to be placed directly online for anyone to read and draw upon, fora for further discussion and comparison, etc.
The Doegen Records Web Project is a quintessential and celebrated example of how the internet facilitates Gaelic dialect study. (Source: Universität Duisburg-Essen)
Sadly, none of the above two paragraphs applied to Dublin Gaelic (or indeed to virtually all of the dialects spoken in old Leinster). Aside from short examples recorded early on coincidentally as general examples of Irish speech by curious travellers, it was not put to pen or paper. Most of its speakers were illiterate in it (literacy usually being acquired in English) and did not, as far as we know, write it; even once the Gaelic revival of the late nineteenth century had taken off, interested types from Dublin often cycled the 101km north to Omeath in Oriel (see map below), which they presumed to be the nearest Irish-speaking area, rather than the 21km south to investigate what may have been a tiny residually Gaelic-speaking community in their midst at Glenasmole. Ironically, Omeath Irish is an Ulster dialect.
Omeath is point 65 on Heinrich Wagner's Linguistic Atlas and Survey of Irish Dialects (1958). The image shows pronunciations of brochán 'porridge' in Omeath, Tyrone (66) and West Inishowen (68). Note all three show significant weakening of -ch- (in Omeath, -ch- disappears; in Tyrone it is barely audible; in Inishowen it is a voiceless glottal fricative [h] like English 'high') and short vowels on the dimunitive ending. These are characteristically Ulster features, as if written bróan (65), bróthan (66) and brothan (68). Rathlin is point 67. You can read more about Rathlin Gaelic, which is very divergent, here.
It seems unlikely (m)any of these enthusiasts knew Gaelic might have been spoken natively (if vestigially) in Co. Dublin: even specialists of Irish - who did crucial work fine-combing Ireland for examples of every last native dialect they could find - overlooked whatever might have remained of Glenasmole Gaelic. So, whilst we - thankfully - even have sound recordings of the Irish of Antrim, Armagh, Derry, Tyrone et al we sadly have none of the Irish of Greater Dublin.
Put simply, the Irish of Dublin was not recorded in any coherent or practical sense.
So that's it? I came all this way for nothing?
The traditional Gaelic dialects spoken in what is now Greater Dublin have nonetheless left scattered, tantalising traces: a handful of passages and word lists, believed to have been taken down relatively phonetically in English orthography in Dublin whilst Irish was spoken natively, although varying greatly in date (from between the sixteenth to twentieth centuries); a significant body of Gaelic loanwords in collections of the English of Greater Dublin, in what examples we have of Fingalian (a descendant of Middle English spoken in North Co. Dublin, itself barely recorded and probably passing in the nineteenth century); quite a few manuscripts written by local Irish speakers whose spelling betrays their dialect; a corpus of many hundreds of place-names in the Greater Dublin area, also varying in date, which perhaps show how Irish was pronounced locally and how this varied subtly from place to place; a few very specific references from observers about the properties of Irish spoken in the area, and a few other sources that represent borrowed Irish words, in what is for the most part their traditional local Gaelic pronunciation, in English.
By systematically comparing and contrasting the forms of these words and sentences with identical and similar words and sentences in other dialects, we are able to identify certain distinguishing features that the Gaelic variants of Dublin were likely to have had, both in common with other dialects and separate from them. (Of course, as Dublin Irish was never recorded systematically and properly linguistically from actual native speakers, any conclusions must remain tentative, but the comparative technique is standard practice in historical linguistics.)
Also fortunately, aside from the tiny and imperfect sources we have for Dublin Gaelic, we also have varying amounts of evidence for the dialects surrounding Dublin, including (in order of the information we have) the Gaelic of Kildare, Wicklow (extremely little), Wexford (slightly more, plus very many loanwords in Wexford English, in which the late great Diarmuid Ó Muirithe took a characteristic interest) and Meath (i.e Seosamh Laoide's Sgéalaidhe Óirghiall (1905) here and an extensive fascinating naturalistic dialogue in both Irish and English here, although it seems from various details in the excerpts that the latter may in fact be in South Armagh Irish), so we have a good idea of the key features of the dialects Dublin Irish was surrounded by. By looking at where Dublin sat on this continuum we have hints of what features it may have had, and in turn the role it may have played in that continuum. (In particular, the fairly extensive records we have of Meath Irish are of great comparative use for the study of the Irish of at least North Co. Dublin, although it must be emphasised here that the Meath material is for the most part from the far north of the county, which is very much aligned with Oriel/Ulster norms.)
I am extremely grateful to Ciarán Dunbar for supplying some linkable resources on Meath Gaelic via his own blog here, and to the late Professor Dáithí Ó hÓgáin, Emeritus Professor at UCD, for the extensive samples and summaries of the Gaelic of Dublin, Kildare, Wicklow and Wexford (among elsewhere) provided in his seminal work Labhrann Laighnigh (Coiscéim, 2011), which you can and should buy here. A little further back in time, we must also thank Seosamh Laoide and Eugene O'Growney for providing us with the majority of material we have from various points in Co. Meath.