One suggestion is that the name Polnoon is a corruption of the old Scots word 'poinding', meaning a ransom, however the word 'Pol' may instead refer to a pool in a river, such as is present below Polnoon's site. Blaeu's map of circa 1654 records the castle as 'Pounuyn' and Punone is the name applied in a 19th century Montgomery family history.

Illustrations of the castle are scarce, however John Ainslie's 1799 survey of the Eglinton Estates includes a vignette of the Polnoon Castle ruins. Timothy Pont's map as published by Johan Blaeu in 1654 illustrates Polnoon as a central single storey house with a flanking building on each side, joined to the centre by a fence-like structure.

The motte lies about a mile south of Eaglesham church, sitting on a promontory with views of the surrounding countryside. The mound is rectangular, 30 by 22 m base, 18 by 10 m top, at least 4 m high and may have been built for an earlier castle of the Montgomerys, for the later structure encased the castle mound itself. The north-west corner of the tower, and the north range rise from the bedrock 3m below the base of the motte.

On the east side are indications of a ditch that ran across the promontory. The main entry to Polnoon may have been to the east of the enclosure as the range on the south extends in that direction. The castle remains do show signs of post-medieval use, possibly as a free-standing dwelling. There are traces of a battered plinth along the south wall.

The castle experts MacGibbon & Ross record the castle name as 'Palnoon' and only refer to the site, describing it as strongly sited on the summit of a steep conical mound'...and ..'fragments on masonry here and there. Today (2011) large chunks of masonry appear almost randomly scattered across the site and in the Polnoon Water; very little remains in place as recognisable structures. On the west side, the ground falls steeply to the Polnoon Water, which forms a confluence with the White cart nearby.

There is no sign of a bailey or outworks on the east. The unusually chaotic condition of the masonry prevents any clear attempt at a plan of the ruins. At the north west corner of the mound is the angle of what must have been a massive wall. At the base of the slopes at the south east end is a stretch of walling 1.6 m thick, possibly part of a courtyard wall.

The Montgomery armorial arms panel would have originally been set in a recess above the entrance to the castle, possibly placed there during the 17th century improvements; it was however removed at some point and can now be seen above the door of the old Cross Keys Inn in Montgomery Street, Eaglesham. This coat of arms is a combination of the arms of the Montgomerie family (the fleurs de lis) and that of the Eglinton family (the signet rings or annulets).