The B4580 is a medium-length road in the area of the England/Wales border. Except for the eastern end, which was part of the A495, the road was originally unclassified, gaining its number in 1935.
The road starts at a roundabout on the A5Oswestry bypass, where it also meets the A495, and follows the old route of the latter into town. After going under the railway line a T-junction is reached on the B5069 and we turn left and follow that into the town centre. The route in the town centre is not immediately obvious, with roads in the one-way system being numbered variably as B4579, B4580 or B5069. On reaching Morrisons we are forced left onto Oswald Road then turn right into King Street. At the end we cross Beatrice Street onto Castle Street. Traffic heading the other way simply follows Beatrice Street to become two-way again at the other side of Morrisons.
At the far end of Castle Street we TOTSO right onto Willow Street. The B4579 soon turns off to the right and then we leave town. There's a straight run across the fields before we cross the old racecourse and then descend steeply to cross Offa's Dyke and the River Morda. After going over a ridge we enter Rhydycroesau where another stream crossing in the village centre takes us into Wales. It is presumably coincidence that the road narrows at this point.
We follow the Cynllaith downstream for a couple of miles and go through the centre of Llansilin. There's a TOTSO right at a crossroads and the road narrows further, becoming little more than S1 as it follows a winding - but not excessively hilly - route west. We go through Efail-rhyd then cross the Afon Iwrch before running into Llanrhaeadr-ym-Mochnant. We enter town along Park Street and reach the market place, where we TOTSO left.
The road remains S2 as it leaves town and follows the valley of the Afon Rhaeadr. Soon there's a right turn signed as the continuation of our road but in fact that is unclassified and the B4580 continues ahead to a TOTSO on the B4396. Following the road strictly requires us to turn right here then left at a crossroads, where the unclassified road mentioned turns right. Now a wide S1 again the road heads south and crosses the Afon Tanat before widening to climb out of the valley. After a hairpin we go through Rhos-y-brithdir then continue to end at a T-junction on the [[B4391].