One way of reaching this summit is from the south starting on the R497 at R91326123 A. Walk straight up the tarmac road heading NE until a gate which is at around 325m which is above most cultivation. Although the OSi map shows a track invitingly leading towards a forest and forest edge road, don't take this unless you are wearing wellies. Instead head directly towards higher ground and eventually to the summit. As of 2011 there is a high mast to the right near Reisk on the map which could well be a test for a future windfarm.
The unmarked top is flat and not very exciting though its position is good for views of much of the Shannons such as Mauherslieve, Keeper Hill, the Silvermines and the eastern Shannons Benduff-Devilsbit ridge. Linkback: https://mountainviews.ie/summit/805/comment/5564/

I climbed Cummer using the the roadway shown on the OS map. This leads up past 3 recently built bungalows. Straight ahead is a hill with a large mast. This is the neighbouring hill of Reisk. Head up the laneway towards Reisk. The track swings sharp left and gets narrower and muckier. As Simon3 says wellies would be useful. The track enters the forest by a broken down gate. Keep right here and follow the edge of the forest up to the top of the ridge and then left to the flattish unmarked summit. The views in all directions are excellent. Lough Derg was visible to the north. Allow 1 hour for the return trip. Linkback: https://mountainviews.ie/summit/805/comment/6597/

Update based on conditions experienced in 2022. You can drive up the lane and there is space for one or two cars at R91728 61731 B. There is also plenty of parking space at R91477 61946 C if you are willing to drive a very narrow boreen with some briar contact. The roadway between R91728 61731 B and R91477 61946 C was in good condition albeit with grass in the middle.

On the accent I walked diagonally from R91728 61731 B to R91746 62495 D and even with the aid of poles I found the ground conditions very heavy going with high grass, heather, lots of grass mounds, a few gullies and some deep water pools. After arriving at the top with one boot full of bog water I opted to descend through the mature forest keeping in one or two rows of trees from the eastern side of the trees. This turned out to be a very pleasant decent and there is evidence of others taking this route through the mature trees. A much safer route even for experienced ankles while the trees are mature. Linkback: https://mountainviews.ie/summit/805/comment/23507/