Here’s what the little Manx Museum and National Trust plaque next to the tomb says:
CASHTAL YN ARD
REMAINS OF BURIAL SITE OF NEW STONE AGE CIRCA 1800 B. C.
ORIGINALLY COVERED BY GREAT CAIRN OF STONE, WITH SEMI-CIRCULAR OPEN FORECOURT AT WESTERN END. A PORTAL FROM THE FORECOURT LED TO FIVE BURIAL CHAMBERS WITHIN THE CAIRN.
That about sums it up.