Not necessarily. In the case of link roads a route number can be shown without brackets - see the "Proximity Rule" in Chapter 2 of the Traffic Signs Manual. While it refers to local roads specifically, I can see it being construed for use here (which is why heading towards the M50 signs indicate so, and vice versa for the R139, though I'm led to believe the blue route confirmatory signs and the sign for the Clonshaugh Rd roundabout are a mistake).

As for the motorway link, is there somewhere where it is specifically stated that they indicate motorway restrictions in place? My understanding of the tag from the OSM Wiki is that it isn't necessary the case.