The usual type of small Breconshire stone circle comprising of low stones.
Experts believe it to be late Bronze Age or in use for a very short time because of the looseness of some of the stones.
It is the only fully excavated stone circle in the old county. This work was carried out in the early 1940s shortly after the MoD took over the land. Since that time many of the stones have gone missing.
Until the discovery in 1994 of a stone circle further north, it was previously the most northerly of the concentration of stone circles in the western part of the old county.
The bleakness of the upland heathland makes the feel and views from Ynys Hir similar to those in the nearby Mynydd Bach and Nant Tarw examples, ie. wind, no trees, sheep, marshland.
To visit, as Kammer has posted below, will entail contacting SENTA (Sennybridge Training Area) for written permission. They do have a website, and parts of the firing range are being opened up to public access.
However, Ynys Hir will never be fully open as it is located in the central impact area (all ranges for safety purposes aim into the centre of the range).
The best time to visit is usually August when the range is least in use.
I had an excellent visit which involves:
Electronic security gates
Briefing (don't pick anything up or try to talk to soldiers as you may become involved in an exercise)
Having to use X Range Exchange to telephone back to HQ when I was on and coming off of the range.
The circle is a good 20min + drive away from where you have to sign in and pick up your car pass.
From X Range Exchange to the circle is a 10min + walk up an old unsurfaced track (muddy in wet weather).
Sadly, on my visit quad bike tracks were visible running through the circle.
This site is within on an MOD firing range, so unless you have a death wish, you'll need to get permission to visit it. There's more information on the Megalithic Walks page, linked to below.