Teach yourself County Down Irish!
How? Well, there is actually a text book!
"An Introduction to the Irish language" by the Rev. William Neilson, although much of it was probably written by Loughinisland scholar, Pádraig Ó Loingsigh / Patrick Lynch. The first edition was published in 1808.
The book is available in full on Google Books, HERE.
To what extent does the book represent vernacular County Down Irish?
It is of course a pre-standard Irish book, but it some ways it is remarkably 'standardised', for example, the usage of the negative particle 'ní' instead of 'cha'.
It is very certainly quite similar to Oriel Irish but it contains little 'patois'.
It does contain a number of words ad phrases however which we must assume to be authentic County Down Irish.
In short, Neilson's grammar is based on the Irish of County Down, likely to be Pádraig Ó Loingsigh's own dialect but the Irish is 'corrected' to an extent with regards to dialect-isms, in addition, it appears to contain a degree of classicalisms, added for effect.