O'Daly's Bridge, Blackwater River.
Simple Trad, a near drive-by. There's room to pull in a car on the Meath side of the bridge and walk to the middle of the bridge to find the cache.
The actual most southerly point in Ulster is about 750 mts south down the river, The Blackwater, but it's private property (fields) with no access to place a cache there, so this is the most southerly accessible point.
The river is also the border between Meath, Leinster & Cavan, Ulster.
