Often done in conjunction with the nearby Slieve Beagh SE, as this is a county highpoint. Slieve Beagh is an unremarkable top marked by a small hummock in a large moorland.
One approach is to drive to a junction about 2.3k NE of Knockatallan at H571404 A and take the minor road on the left going NW to Barratitoppy Upper. If a sign still indicates that this road is for "residents only'" confirmation has been obtained from the Knockatallon Community Centre that it is ok for walkers to drive up road to access Slieve Beagh.
Park carefully where a gate crosses road at H554415 B about 2.3k along. When the track ahead forks, bear left and at its end head NW for 1.4k over bleak moorland. On the way cross Pt 365 and then go on out to a small peaty hummock H5318543567 C - Slieve Beagh SE Top, beside tiny Lough Sallagh. Continue 1km west to reach the main summit.It's a desolate area and the need for navigational accuracy in poor visibility must be stressed. Return by way of ascent.
Alternatively park at lay-by at H562 432 D. 300m south of this a track heads NW giving 700m of good walking before the terrain becomes rough and boggy. (It is also possible to drive up this track for several hundred metres as it is used for turf-cutting access). At end of the track turn west for 3km. Choose higher ground for easier underfoot conditions.3 hours for return journey. Linkback: https://mountainviews.ie/summit/866/comment/5625/

Took a trek up Slieve Beagh this morning as part of my County Tops task. What can I say? It's desolate, barren, bleak and moody, a veritable oasis of emptiness! Pretty much took the same path as murphysw above. From Knockatallon, head out by the Slieve Beagh hotel and take the road to the left signposted 'Three Counties Hollow'. Park up at the electric gate and follow the path up by Lough Antracier. Veer left where the path splits in two and after a couple of hundred metres, the path peters out and it's onto the open Bog. From here the going gets purgatorial, to steal a phrase from Paddy Dillon. Grass and heather is waist high in places and the ground is as you would expect, very heavy and boggy - I went knee high into water at one stage. I hadn't a GPS but walked out as far as Lough Sallagh so passed over the highest point of Monaghan at some stage. Due partially to time restrictions and partially to the fact that I could see rain on the way, I decided not to continue onto the summit of Beagh itself and retraced my steps. Warning: I would not recommend heading up there in poor visibility without a map/compass or GPS as I'd imagine it could be very disorientating. Linkback: https://mountainviews.ie/summit/866/comment/3456/

Apologies in advance that this is so long! I don’t know when to stop; hopefully though someone might it useful. This is an area of upland I'd been intrigued by for some time and finally attempted it earlier this year, bringing my much suffering canine companion Susie with me. I'd already walked part of the Monaghan section of the Slieve Beagh Way (SBW), around the Penal Cross, but was keen to reach the summit 'where three counties meet'. I took an alternative route to the previous posters, included in a nice set of route cards called Walk South Ulster (if anyone wants a copy I can scan & email). I approached from the R186 Monaghan/Clogher road; Knockatallan and its walks are signposted a little north of Tydavnet. I passed the minor road signposted 'Three Counties Hollow', then the Slieve Beagh Hotel and at the main crossroads (signed Scotstown left & Roslea/Fivemiletown straight ahead) I took right up a minor road uphill. After almost 5kms you cross into Co. Fermanagh (speed limit signs give it away) and shortly after there is a carpark on the right along the SBW at Muckle Rocks, part of Mullaghfad Forest. Walk along the track into the forest, where you almost immediately take a right, then after about 1km take the track branching off to the left. Follow this north and when it ends continue across the start of the boggy land until the end of the line of forestry on your right. The card says from here 'turn slightly to the right onto the brow of the ridge' and when on it 'turn north again and walk towards the small mound in the distance'. Unfortunately I didn't quite pick out these features in the mostly nondescript, boggy landscape all around and ended up veering much too far to the right, northwest of the line of the forest, until I met a wire fence and small stream just beyond it (more on these later). While I was eventually able to find my way again, I would basically try to just head due north at the end of the forest. After some time you see a small clump of forestry ahead, head towards this and you come to the reedy lake known as Shane Barnagh's Lough and to the east of it, a small rocky hillock, Shane Barnagh's Stables. These features ordinarily wouldn't warrant much interest but I was delighted to come across them I knew I was on the right path, plus they broke up the monotony of this desolate landscape; also there's the legend of the 17th century cattle rustler and his treasure apparently buried at the bottom of the lough! The small summit of Doocarn is the highest point in the area, behind the clump of rather unhealthy looking forestry up ahead. To reach it, skirt to the right of the foresty, cross the fence and head north then left. I must warn that the summit area is very uninspiring; in fact there were two very minor bumps and hope I walked up the right one! The views aren't bad though: I could see Cuilcagh off to the southwest and the Sperrins to the north, and closer the wind farm between F’miletown and Fintona. Linkback: https://mountainviews.ie/summit/866/comment/3873/

PART 2: From the summit follow the same fence down until you come to the depression (again, very uninspiring) known as Three Counties Hollow where Fermanagh, Monaghan and Tyrone meet (it would be nice if there was a small marker to indicate exactly where this point is, seeing as its one of the key attractions of Slieve Beagh... I did however see a sign saying 'No shooting'!). From here you can see, to the northeast, the small rounded Lough Sallagh, which is in fact split between Tyrone and Monaghan and is worth a look. The route card recommends upsloping onto the ridge of Eshbrack (Monaghan) before finally returning to the start point but I was conscious of time and of again getting confused in the featureless landscape, so I decided to head south along the fence as I knew from earlier that it led back to the (far side of) the forest. After a while the same stream I had seen earlier could be heard then seen, this time to the left of the fence. I walked for seemingly ages through this unmerciful, reedy terrain (bad idea to wear boots with large heels) alongside the left bank of the stream, with the forest never seeming to get much closer, cursing this desolate landscape and thinking to myself that I'd finally got this hiking malarky out of my system! Finally, however, I met the forest to the right and I crossed the stream and fence and walked south along the track at the forest edge, which was much easier and very welcome terrain. Heading south here I noticed a pretty little waterfall in the stream. Incidentally, the stream forms the border between Monaghan and Fermanagh (further downstream, I had crossed it in the car just before the car park) and because it is so narrow here you can get some great quirky photos of you literally straddling the border! After a time you see a small footbridge crossing the stream on your left (this is part of the SBW and I would've crossed it eventually had I followed the route card) and here there is a path into the forest on your right. Follow this for about 1.5kms back to the car park; passing, on your way down on the right the turn-off you originally took out of the forest. Overall, Slieve Beagh was definitely memorable if not altogether enjoyable; it's probably one of Ireland’s least known areas of upland so it's nice to know you're one of the few to actually bag it (I literally didn't see another soul for hours). Sadly though, because the summit area is a huge rounded mound the views aren't generally that good. Also, I would stress again that the terrain is desolate and tough going; in poor visibility I can see how you could get disorientated and panic. I realized I had dropped the route card at one stage and spent an anxious several mins retracing my steps. I also belatedly realized I had lost my jumper that I had tied around my waist, which I never retrieved. It's blue and lilac and if anyone finds it up there please contact me as I will be delighted to see it again!! Linkback: https://mountainviews.ie/summit/866/comment/3872/

In terms of height this county high point is certainly unimpressive, but as a challenge this lonely, brooding, godforsaken area certainly punches above its weight. Without a GPS or map and compass it would be easy to become disorientated as I found today when I climbed this in mist and drizzle. Getting there through the maze of north Monaghan roads is confusing enough! You can park at about H552418 E before an electric gate bars your way. However there is a rough road until H546424 F which you can walk. At this point the Slieve Beagh way points to the right, but you need to take the track to the left. This track soon peters out and its about 700m over wet bog to your next target which is Eshnabrack (spot height on OS map atH538431), the highest point in County Monaghan, though there no point to speak of and everywhere around you is around the same height, but you can take comfort in the fact that you are indeed the highest person in Monaghan as no one else is dumb enough to be up here without a valid reason! I then made my way over to Lough Sallagh (H532437 G) to break the monotony and while I was there decided to head over to the summit of Beagh itself (H524436 H) which at least has a boggy mound to recommed it. Linkback: https://mountainviews.ie/summit/866/comment/3320/

Slieve Beagh wasn't considered to be up to much in the 19th century, judging by the Penny cyclopaedia of the Society for the Diffusion of Useful Knowledge (1839), which included the following on Monaghan:

"The whole county is hilly, but the hills seem as if scattered in an irregular manner, without forming ridges or chains. The principal heights are Slieve Baugh or Slieve Beagh and .. The Slieve Beagh Mountains form an uninteresting waste along the boundary of this county and Tyrone, and are sterile without being picturesque."

Perhaps modern tastes have changed. Certainly as visitors we valued the quietness of the place, an Area of Special Scientific Interest in Northern Ireland and a National Heritage Area in the Republic. As paulocon says, an "oasis of emptiness". Linkback: https://mountainviews.ie/summit/866/comment/4747/