Every road has a number. If you can find a better one, we can tag it. In addition, many of the legal documents are out of step with reality on the ground (see the other edit of yours that I just reverted). We have seen all these documents and done a lot of soul searching to make the most reasonable tagging decisions we can. The N52, which legally isn't supposed to exist north of Dunleer, has very new signs for it all over Dundalk and on the M1. This is Ireland and a lot of this stuff is sloppy. So again, numbered regional roads provide connections between higher road categories. R139 is there to strategically  connect to the M50 and M1. I'm sure we can agree that its strategic purpose is _not_ to end a few hundred metres short of the motorways to terminate on an unclassified road. The status of the spur is troublesome to work out, but what's clear from the newest signs (those erected when the junction was freeflowed) is that the roundabout and spur road are no longer under motorway restrications (the spur previously was under restrictions, and I think that's why the legal docs describe the R139 as terminating on the spur). But now the spur is detrunked, the only reasonable classification is R139. And if it is to be classified as highway=secondary, it would need to be R-something.