Go to osi.ie mapviewer, zoom in to the area in question, use the overlay slider to bring through the 6" or the 25" old map showing the county boundary.
If you click on the link below it will display an area around Urlingford and you can clearly see the county boundary.
[broken link removed]
A word of warning I have encountered inter county rivalry along this border which resembled trench warfare on the Western Front in the 14-18 war!