"The session at McTarvish's was mighty, last night, and the crack was only ninety!"...
This is not what it appears to be. The sentence above is describing a sort of "chemistry" that occurs durring a "session" or gathering of Irish musicians, in which, through the right combination of music, conversation, drink and people, there was achieved an electric and mysterious buzz in the room, referred to as "crack". The phrase "only ninety" isn't marginalizing the crack, but is used, instead, like "completely". Crack is always hoped for, but must always be only the spontaneous ignition of the right elements, generating a figurative static sizzle in the atmosphere. These gigs or sessions usually take place in Irish pubs, the very sanctum of traditional Irish music.