Does it need to be a forest? The Huguenot cemetery on Merrion Row (just down from the Gresham Hotel) seemed to have loads of bluebell drifts when I passed by there earlier today.
Edited: This photo seems to have been taken around this time last year by a tourist. The colour's a bit washed out, but it gives an idea of the bluebell content! At least, they looked like bluebells from the road (I wasn't walking). They were certainly bluebell colour - far more vivid than in the linked photo.
I was in the Botanical Gardens this morning. There is an area near the Laburnum walk with thousands of them looking lovely. The area is about the size of the pedestrian area at the top of Grafton Street where the buskers hang out. Still a lot of tulips in bloom and beautiful Peony Rose also.