It's at the end of a short cul-de-sac called Clonmel Street which is on the opposite side of the road to the Garda Station close to St. Stephen's Green.
No the other side. If you are heading down Harcourt Street towards St Stephens green it is about half way down on the right hand side. Enter via Stable Lane. Correction Circle is right it is clonmel st. Got the two mixed up. sorry for any confusion.
There's also an entrance from Hatch St, if you're coming down Harcourt St it's the first turn on the right immediately after Luas stop, gate is on left hand side of street.