Mullaghcarn had the temerity to sprout a South Top during the recent Arderin Beag shenanigans, and it was with this as the prime objective that myself and Indy the Three Legged Wonderdog set off from the B48 at the foot of the forest road oft used (especially on Cairn Sunday at the end of July) for the ascent of the main summit. Indeed, doing nothing more than keeping to the tarmac (rather than diverging onto any of the more 4WD tracks conspicuous on OSNI Sheet 13) will inevitably bring you to within 50 metres or so of Mullaghcarn itself (turn off left just shy of the transmitters).
From here the route to the South Top follows the fence south-west; I took advantage of the stile adjacent to the summit as getting Indy over barbed wire fences can be fraught) and followed its far side. This is pathless going that becomes typically Sperrinesque in the depression (moisture in abundance) before a short easy rise to the top, which lies at some point on a natural embankment surrounding a conspicuous peat hollow.
I returned the same way. Its probably quicker to get over the fence earlier and then go to the left of the transmitters (but like I said, three-legged dog and fences...I wanted to use the stile again). If tempted to use a full-on short-cut to meet the route of ascent lower down then bear in mind the intervening ground is recently felled forest and as such is probably a right barrel of laughs to traverse.
(Note that the track should be c.20 minutes longer: the descent from the main top was largely run).