The name Cruachan is the diminutive form of Crucah, and is even more common in its usage than the non-diminutive form.
The word itself "means 'Little Rock'" (Flanagan, 65).

“Cruachan is traditionally said to be the inauguration place of the Kings of Connacht.  There are a number of monuments spread over an area of about two square miles.  These include a large mound, a number of differently shaped enclosures and some ring-forts.  One of these contains a standing stone alleged to mark the resting place of the last pagan king of Ireland” (Bord Fáite Éireann).

'The Pillow Talk'

      Once when the royal bed was laid out for Ailill and Medb in Cruachan fort in Connacht, they had this talk on the pillows:
      ‘It is true what they say love,’ Ailill said, ‘it is well for the wife of wealthy man.’
      ‘True enough,’ the woman said.  ‘What put that in your mind?’
      ‘It struck me,’ Ailill said, ‘how much better off you are today than the day I married you.’
      ‘I was well enough off without you,’ Medb said.
      ‘Then your wealth was something I didn’t know or hear much about,’ Ailill said.  ‘Except for your woman’s things, and the neighbouring enemies making off with loot and plunder.’
      ‘Not at all,’ Medb said, ‘but with the high king of Ireland for my father – Eochaid Feidlech the steadfast […].  My father gave me a whole province of Ireland, this province ruled from Cruachan, which is why I am called “Medb of Cruachan.”  […]  When we were promised I bought you the best wedding gift a bride can bring: apparel enough for a dozen men, a chariot worth thrice seven bondmaids, the width of your face of red gold and the weight of your left arm of light gold.  So, if anyone causes you shame or upset or trouble, the right to compensation is mine,’ Medb said, ‘for you’re a kept man’ (The Táin, 52-54).