Beinn Lagan from the Smiddy - the easiest way.

From the bridge walk up the forest track for about 200 yds then turn right onto another forest track. Carry on up the gently rising track and turn left at the first junction to another track heading up more steeply. After a while it flattens out then rises again. There is a point where a small stream cuts across the track and you will need to jump or scramble to pass. The track surface gets worse as you climb, but it is easily walkable.

You will come to a three-way junction with a better track that runs around the hill. Bear left, and then almost immediately take the "quad bike track" heading more steeply up towards the top of the hill . You can keep on the track all the way to the trig point. It does take a bend to the right part way up and you may be tempted to scramble straight up. I found it was best to stick to the track though, as it bends round a small bluff and climbs gradually all the way.