To yield or not to yield!

Coming out of IKEA on Saint Margarets and heading onto the R108 northbound, one passes traffic lights with amber arrowed filter lights followed by a yield sign, as shown on the google streetview link below:
https://www.google.ie/maps/@53.4053793,-6.2656252,3a,75y,52.62h,78.06t/data=!3m7!1e1!3m5!1szLDK2e0Kfc71DQ9cVDkx4Q!2e0!6s%2F%2Fgeo2.ggpht.com%2Fcbk%3Fpanoid%3DzLDK2e0Kfc71DQ9cVDkx4Q%26output%3Dthumbnail%26cb_client%3Dmaps_sv.tactile.gps%26thumb%3D2%26w%3D203%26h%3D100%26yaw%3D255.68968%26pitch%3D0%26thumbfov%3D100!7i13312!8i6656


The next link shows the overhead view with Saint Margarets Road on the left and the slip to the northbound carriageway of the R108.
https://www.google.ie/maps/@53.4056112,-6.265253,135m/data=!3m1!1e3

The question I have is why do cars coming from Saint Margeret's Road onto the R108 NB have to yield to traffic on the other lanes on the carriageway. The slip lane from Saint Margarets continues as a new lane onto the R108. As far as drivers already on the R108 at this junction are concerned, this is a new lane and it would seem logical that they would need to yield to Saint Margaret Road drivers who were in this lane first however road signage suggests otherwise.

The next link shows the viewpoint from a driver already on the R108 at the slip road.
https://www.google.ie/maps/@53.4053819,-6.2653686,3a,75y,349.79h,92.12t/data=!3m6!1e1!3m4!1sr0TloyB0Q0B_OvBfVTQhYA!2e0!7i13312!8i6656

As soon as a car has passed the solid white line at the yield sign, has he not got right of way in that lane? Going by the cross hatchings, it would be almost another 10 metres after this point before a driver on the R108 could move left into the slip lane in order to access the M50 SB, at which point the car coming from Saint Margarets Road is undoubtedly under no obligation to yield.