Finbarr,
I spent an hour rambling around the virtual trackbed of your desired location, and theres no doubt but even now it's one of the most scenic railway layouts on this island and would make for an excellent layout. Unique bridges, weird level changes, cast iron bridges across rivers, riverside track - all potentially excellent diorama material, and relatively easily achieved. I'm going to go against the advice up top and recommend that you steer well clear of narrow gauge and brass kits, after all you are a complete novice. This is your layout. It's not for exhibition, so lay the track in 00 off the shelf, and as you develop as a modeller and gain confidence you'll happily dip into the more specialist areas. But for the moment get out a sheet of A4 paper, mark it out in large 1 foot squares, and start scribbling your 16' x 6' space, with the chosen area in mind. Don't be so hung up on achieving a museum standard replica of CB& PR - As you said yourself your better at the Art than Maths, so allow some creative space. Perhaps the layout might be a large river side sweep of track terminating/running through a station like Bandon.
If people can recognise the area straight off, even if it is fictitious, that should be more than enough to offset the desire for perfection against an enjoyable working model.
Go the Ordnance Survey Site and check out the historical maps of the area http://maps.osi.ie/publicviewer/#V1,591271,743300,0,10
From a Permanent Way viewpoint, it's not very exciting operationally, so perhaps a bit of "creativity" is needed. As one of the lads said earlier, if you multiply your actual space by 76.2, you'll likely find you can fit bugger all in 16 feet. If you were to model Connolly Station for instance it runs to 36 feet, and that's just as far as the cast iron bridges over the river.
Richie.