Traffic levels on the N3 are low on all but the sections close to Dublin. The M3, which opened in 2010, seems overspecced for such levels, especially beyond Dunshaughlin.
In 2010 in the graph above the traffic counters switch from the old road to the new one.
It seems that the new road has not succeeded in pulling a significant percentage of vehicles off the old road.
As of 2016, around 30-50% of the total traffic on the M3 and old N3 is still on the old route.
For example, at Dunboyne, 20000 vehicles per day travel on the old N3 and 18000 on the M3. This means that only 48% of the total traffic is on the motorway.
At Tara, 30% of the traffic is on the old road, and south of Kells, it is back up to 41%.