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!
There are numerous online mapping systems that will provide this (as mentioned above), but if you need to get more detail it's easy enough to get the data (e.g. geo-referenced county boundaries and background [or aerial photography/orthophotos] GIS mapping).