How to find Kilneuair Church and Fincharn Castle
The following images show modern maps of the area..
Do go and have a look!
The starting point is ideally Oban, then proceed south on the A816 almost to Kilmartin, then turn left (North East) on to the B840 towards Ford (Carnassarie Castle is on your right on a hill when you reach this junction).

You should follow the road round to the right through Ford village and then along the south side of Loch Awe. The first site that you reach is that of Kilneuair Church (4km / 1.5 miles out of FORD). The ruin is 200 yards south of the road, on a path which is sign posted to Loch Fyne (the church is on the east side of the path). You can park in a clearing on the south side of the road, approximately 150 yards east from the path.
Fincharn Castle is a further 2km / 0.75 miles east along the B840, 400 yards north of the road. Access is across a field, which you enter through a gate immediately opposite the entrance to Finchairn Farm (you cannot see the castle from the road - so watch out for Finchairn Farm!). Parking is not easy but you should be able to stop on the verge without blocking the road.

Map references are:-
Kilneuair Church - NM889037
Fincharn Castle - NM898044
The map which I used is Ordnance Survey Landranger (1:50,000) sheet no 55. Both the church and the castle are marked on this map.
Good Luck!