[RWD version 1 ] There are various routes published on MV for reaching Croaghconnelagh however they assume that the various streams and rivers are passable.
This is how to reach the top when the water is up. But it is NOT guaranteed and definitely has a dangerous stream crossing.
Start from the N15 at the prominent carpark. Cross the Lowerymore River by the bridge and immediately head up the valley keeping close to the river on its western side.
There is, as of 2019, an overgrown but quite definite and very helpful track along the riverside. Make sure to find this as it will make your walking altogether easier. It is generally about 5m from the river.
Although not shown in full our outward track went higher which took us through very boggy ground covered in long grass and heather which we found was energy sapping to move through.
When the river track reaches a further stream that descends in a NE direction from the higher ground around Croaghconnellagh it is time to turn left and start climbing.
This stream is not shown on OSM derived mapping but is on the OS 1:50k. The track continues for a bit though eventually peters out. It is at this point that you have to choose one of two ways of proceeding. Up the left side of the stream or the right. The 1:50k suggested that the land on the left side, which would be the more direct route to the summit, was steeper. In fact it is much steeper and would have been very difficult in the conditions. The right side of the stream is the preferred way of going and that is what we did and what is shown on the map.
However this means crossing the stream which was a dangerous torrent. This was just possible on the day. However I wouldn't assume that this was the case on another occasion. Also I would assume that this stream could rise very fast in response to heavy rain and become impassable, making a return trip very hard. Not all members of a party would find crossing possible.
We proceeded upwards as shown on the track. The terrain continued to be very difficult with long grass etc until around 400m when the underlying granite which is very walkable started to appear in places.
The summit area could be described as convoluted. It's suitable for a difficult mountain leadership assessment using only map and compass and designed to fail as many trainees as possible. There are numerous valleys, outcrops, boggy patches and false summit lumps that have to be negotiated. Also at the top there are at least 3 or 4 cairns on different candidate tops. Given limited visibility we went to all of them since we weren't sure which was the highest.
Return the same way, hoping that you will be able to recross the stream. Going across the Lowerymore River was out of the question. It was 4 metres wide and moving fast.