I have done you some playing around DR268, not least as I am intrigued by the problem.
I found the village of Straw on BaseCamp and found your spot on the B47 / B162.
I then asked BaseCamp, in motorcycle mode, to give me a route from along the B162 close to little road called Corrick, via the obligatory pee stop at the crossroads, to a point on the B47 just at the Mourne river. This it did, routing me exactly as your screen shot shows, along the goat track, across the crossroads, up the B162, then left on another goat track to rejoin the B47. This was a route of 6.8 miles, estimated time 0:15:15.
I then dragged the route to follow your preferred route 1,2,3 ie up the B162, to a pee break on the crossroads, then left on the B47.
This resulted in a route of 6.8 miles but taking 0:10:38.
This confused me as I have BaseCamp set to give me the fastest time A to B. Clearly the second route is five minutes faster than the first, so why didn’t BaseCamp give it to me the first time of asking?
I then changed my preference setting in motorcycle mode, to exclude narrow trails. It still routed me the slower way.
I then changed the mode to RV, which should bring in the avoidance of minor roads automatically. Bingo! It followed the 1,2,3 roads exactly, 6.8 miles in 0:16:59
Switching into car mode, routed me the slower way, as did every other mode, bar RV.
Conclusion:
1. I am at loss to understand why the BaseCamp software takes the slower route over the same distance.
2. There is nothing wrong with your Nav VI, that I can see.
2. My only thought is that, over that short stretch there is a glitch in the mapping or the algorithm.
3. The glitch is repeated in the Nav VI.
4. Had I been creating the route for myself, I’d like to think that I would have spotted the glitch and edited it out; probably without giving it much thought as to why it had happened. If nothing else, that supports my mantra of checking all routes carefully.
I have saved the route, as I think the glitch needs to be reported to Garmin.
If anyone else has some ideas or better still a fix for the glitch, sing up, please.
Richard
PS Here's the Garmin file, saved to Dropbox
https://www.dropbox.com/s/9ozfts6c6n7ikdl/dr268.gpx?dl=0 if anyone wants to play around with it.