Several possible approaches. The first is to park at carpark near Maghera beach G662907 A and head directly west onto open slope, passing an old ringfort and the small Lough Acruppan, where you turn WWS, ascending a gentle slope for 2km, taking you to the summit, incorrectly identified on OSI map as Meenacurrin (the map also incorrectly places Slievetooey 2km further west, in the middle of a broad plateau). The total time to top, 2.5 hours.
A simpler approach may be taken from the SE. Park at the layby G641 891 B (parking for 3 cars). Head up the hillside to the west for a couple of hundred metres, initially steep, then easing, (a couple of fences to cross). Then turn directly north. The terrain rises gently, is pretty firm, short grass with patches of heather, and as you near the summit becoming more rock-strewn. This approach takes just over one hour.
A similar approach is to park in the same spot, but walk a few hundred yards along the road to the paved lane at G639 887 C. This roadway is marked with official 'walker direction poles', so one assumes it is ok to use. The paved bit ends after a few modern farm buildings and becomes a stony track, crossing a stream. From anywhere here you can access the open hillside, where you should simply head directly north towards the summit.
The summit offers first rate views, especially across the bay to the north, and down to Lough Croaghballaghdown, which nestles in its northern slopes. Linkback: https://mountainviews.ie/summit/430/comment/5189/