You could avoid some of the Navan traffic (esp the Dublin-bound) by turning right at the fire station in Navan (heading for Kells), taking the next left, then right at the roundabout heading for the hospital.
Go straight across the junction at the hospital, passing pairc Tailteann on your left, cross over the railway line and continue until you reach the first set of traffic lights at a crossroads. Turn left - you're now in the Beechmount Estate area. Continue straight to the end of that road until you meet the main Navan-Trim road (petrol station ahead of you). Turn right and you're on the way to Trim.
It can be used on the way back too, can be a little more congested, but you're avoiding having to go through the busiest part of Navan town.