This summit has much to recommend it, being high & cliffy, having the heart shaped Lough Ouler below it and being part of the fine ridge from Mullaghcleevaun to Laragh. It overlooks much of the heart of Wicklow.
Other than getting to it from that ridge you can reach it from the Wicklow Gap O0752 0017 A from which it is a simple though stern climb of about 350m or from the Glenmacnass Car Park at around O1123 0303 B. This route crosses the Glenmacnass River, which is prone to fast flooding and goes via the NE top, a minor ridge. Either way it's going to take about 1h30 to 2h round trip.
North of Tonelagee there is Stoney Top before the notoriously boggy ridge towards Mullaghcleevaun. SE there is a sharp descent (untracked as of 2013) to a boggy area which has a path towards the Brockaghs. Trackback: https://mountainviews.ie/summit/33/comment/4793/

On a clear day look east from Tonelagee and you may be rewarded with rather more summits than you might expect.

The nearer summit is Scarr, the hazy summits on the skyline are in Wales.

After some very helpful contributions on newsgroup uk.rec.walking (thanks Paul Saunders et al) the best estimate is that the summit at D is Crib y Ddysgl (aka Garnedd Ugain 1065m), which would make Summit E just to the right, Snowdon (1085m and 155km away). Between E and J is the Nantlle ridge. The summit at J may be Yr Eifl. Trackback: https://mountainviews.ie/summit/33/comment/2604/

Climbed this mountain on the 22nd August 2009. Weather conditions were absolutely perfect on the day. From OS Map No. 56 I parked at location T 099 982 C (the car park at the disused lead works overlooking the Vale of Glendasan) From here I took a northern route direct upto Brockagh mountain, this is the best route to avoid the thickest of the Gorse and Heather. Approx half way up take a north east bearing to reach the top of Brockagh. I have climbed this mountain twice and find it the best route. From the summit of Brockagh it is possible to hike across to Tonelagee. From OS Map No. 56 there are three summits (unnamed) shown between the two mountains, I used these as my way markers across to Tonelagee. These appear to be the best routes across as it is quite marshy and boggy, try to avoid the low-lying ground as it is quite easy to get stuck. Also beware of some holes in the ground leading to underground springs. They are difficult to see until nearly too late but due care and attention is enough to keep you safe. From the nearest summit to Tonelagee located to the south east, gently start making your way north east, avoiding the valley and springs that block your way (this will make sense when you see them in front of you) I proceeded to O 093 018 D (approx halfway between Tonelagee NE Top and Tonelagee itself) this route appears to be one of the easiest to climb from this side of the mountain with the added benefit of a great view of Lough Ouler. The last third is a tiring climb but relatively straight forward as the sheep and mountain goats have marked a route in the boggy ground.
I descended taking a south west route from the summit, basically heading for the wicklow gap car park from the summit. This route is very overgrown with heather particularly as you get nearer the base so a little care is advisable. It is doable but the ground can be undulating. Once I got to the Wicklow gap car park I took the main road walk back to where I parked. Began this hike at 9am and was back in the car by 1.30pm. Perfect weather conditions. Trackback: https://mountainviews.ie/summit/33/comment/4032/

Tonelagee in the centre with Mullaghcleevaun to the left , pic taken from Lugnaquillia in january 05 .The flat section beneath Tonelagee is a resevior on Turlough Hill almost 700m high. The E.S.B built it in the 70,s.
Water is pumped down hill at night to turbines creating electricity.Climbed Tonelagee, with friends, in July 04, from the car park at the Wicklow Gap, which is between Turlough Hill and Tonelagee.Lovely views from the top of Lough Ouler and surrounding mnts.Went from there to Stoney Top and on to Mullaghcleevaun ,then to Mullaghcleevaun East Top .There's a large upright stone with a cross etched on it on Stoney Top, could have something to do with St. Kevin.We went from east top down through the valley and made our way to Lough Ouler.There i found and held a common lizard , the only type of lizard native to Ireland .Same size as a newt , but with a snake like head and forked tongue , not very common.We climbed Tonelagee from the lake and made our way back to the car park. It was a clear day with good visibility, the walk was about 11 miles and took us 7 hours .Well worth doing on a good day. Trackback: https://mountainviews.ie/summit/33/comment/1438/

In 1895 H.C.Hart said of Tonelagee ".. a round mass of moorland, has on the northern shoulder a crater-like valley, containing a tarn, Lough Ouler, and cliffs of schistose, some 400 to 500 ft. high, descending from the near the summit to the margins of the lake." This picture shows Lough Ouler where it feeds into Lough Brook on a bright day in Sept 2002. Trackback: https://mountainviews.ie/summit/33/comment/156/

The trig pillar on the top in Feb 2003. The weather was perfect: it felt a little bit like climbing in the Himalaya (snow and wind). Trackback: https://mountainviews.ie/summit/33/comment/1705/