You can start at Ballaghisheen, a mountain pass V671 792 A. Space here for 3-4 cars. Ballaghisheen is at just over 300m so the climb to the top at around 551m is not a great strain.
There is a sign advising that access is prohibited however the broken fence and tracks uphill indicate access must be frequent.
Head uphill skirting around the mountains east side initially which will being you to the bottom of a deep, wide cleft in the ridge. It is pointless trying to climb up before this as you need to descend into it and climb out of this cleft in any case to access the summit route.
Climb up then skirting the steep edge of a corrie-like cliff (no water though) before heading directly for the flat grassy summit. Linkback: https://mountainviews.ie/summit/364/comment/5123/

A visit to Knocknacusha is probably best seen as a short trip, not as part of a longer route. The sort of place you can try if you want an easy walk, but not without its rewards. You can start at Ballaghisheen, a mountain pass. There's space for just a few cars here, if you are driving. Ballaghisheen is at just over 300m so the climb to the top at around 551m is not a great strain. The summit is unexciting -- there is a small pile of rocks at the top.

The view south is towards Coomura Mtn (666m) over an area of very broken ground with a lot of bare rock. Just right of centre near the top of the picture is a big glacier flattened slab. On the way up to Knocknacusha there's a couple of similar pieces of glacial handiwork to look at. Linkback: https://mountainviews.ie/summit/364/comment/1846/

While you are on Knocknacusha take a walk along the east ridge. You can go over 600m without dropping much. The view is great for so little work. To your right Cloon Lough will come into view between you and the well known Mullaghannatin area. To your left Colly mountain and the southern arm of the Glenbeigh horseshoe. The picture shows the front view including the south side of the Reeks in the clouds on the left, the sharp peak of Broaghnabinnia just right of centre and the lumpy mass of Stumpa Duloigh (784m) to its right. Linkback: https://mountainviews.ie/summit/364/comment/1848/

As Simon has suggested, this works well as a short trip from Ballaghisheen (671792 A), a short boggy and bumpy section giving way to an easy-ish climb. But I think his picture doesn't dissuade any aspirant ascentionists thinking of doing Coomura via this top nearly effectively enough, so that's my excuse for posting a pic looking in much the same direction: it looks VERY impressive from here. Linkback: https://mountainviews.ie/summit/364/comment/3770/

Similar to the other contributors, this is not a single summit day out. I did it after Colly. It's an hour or so with a coffee break. Start at the Ballaghasheen pass (spectacular drive in & out) and head slightly westwards starting at a metal pylon. No broken fences or signs here. Skirt slightly on the west side until the first few rock shelves are climbed. I advise going up on the lefthand side (steep) and down on the left hand side (less steep). The views are superb! Summit is unremarkable. Linkback: https://mountainviews.ie/summit/364/comment/18849/

Pic show summit marked by a small pile of stones. Linkback: https://mountainviews.ie/summit/364/comment/4021/