This was a question that was posed on Reddit and answered by Erisceres (who I know). I’ve posted their response with their permission.
Firstly, it’s best to understand the varieties of Gaelic spoken across Gaeldom (Ireland, Scotland and Mann) as belonging to one large dialectal continuum rather than distinct major varieties. This sets the context in place for Leinster Irish.
It has been said that just a couple of centuries ago, one could travel from village to village, starting in the very South West of Ireland and finishing in the very North East of Scotland; one would have noticed only subtle differences in dialect between each village throughout their entire journey. Thus, it’s best to think of dialects as belonging to such a continuum. The effects of this continuum have largely been lost due to the rapid decline in Gaelic everywhere, leaving behind only a few places we now call the Gaeltacht where dialect still survives.
So, with that in mind, there isn’t really any such thing as Munster, Connacht or Ulster Irish, let alone Leinster Irish. We use these terms as they are useful in broader contexts where certain dialectal features are shared across the specific instances of surviving Gaeltacht regions.
For instance, in Munster, it would be more effective to say West Muskerry Irish or Ring Irish, as the dialects in these two regions have their differences, but share many features due to proximity.
Now we come to Leinster Irish. Dialects spoken in Leinster survived to varying degrees up to certain points in time. In the south, especially in Kilkenny, the dialect survived up until the early 20th c., and was fortunately recorded rather well from the last remaining speakers. Due to its proximity, its features best align with Ring Irish, but have noticeable differences.
In the north, including North Dublin, the dialects are more closely aligned with East Ulster dialects. And the further west, they more closely aligned with East Connacht dialects. Hence the effects of the continuum.
As mentioned with Kilkenny, features from other Leinster dialects were recorded, but perhaps not to the same level. Further research has been done by studying placenames in different locations to determine dialectal features, including in Dublin.
So, we can know a few things about how Irish was spoken across Leinster and with more research we could derive even more information. But someone would have to put in the work to figure it out, and it’s often best to focus on specific areas to avoid being overwhelmed by such a monumental task to cover the entire province.
But thanks to what has been recorded, many resources do exist that can teach us a lot about Leinster Irish. But these resources are quite linguistic and would need a good bit of understanding of Gaelic phonology and morphology, and a general understanding of isoglosses and diachronic developments for better comprehension.
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