This is no blind tasting, this is just a side by side to see what's what.
At 4.3% and 4.4% they are pretty much the same strength. On pouring I noticed an immediate difference in head. The O'Hara's had one for a start while Dark Arts disappeared in a puff of voodoo magic. In fact it was gone before I could turn the camera on. The aroma is also different with O'Hara's having a more lactic sourness with coffee chocolate and caramel. Dark Arts was dominated by chocolate, and more like milk chocolate or at least a weak dark chocolate like Bournville, less coffee and plenty of caramel. It smells sweeter. The taste is also quite different, in fact it seems they do a bit of a switch. From the aroma I would think Dark Arts will be sweeter and O'Hara's more bitter but I found the opposite was true. O'Hara's had a more lactic sourness as I expected and more coffee with lashings of toffee. Dark Arts had a sort of woody vanilla start which progressed to bitter dark chocolate (nothing like the aroma) and some coffee in the middle. Towards the end, a slight sourness came through but it was not as evident as O'Hara's.
As many times as I have had both these beers, I have never tried them side by side so it was great to see them being quite different. I do not have a clear favourite as both are beautiful. I think at a push, if both were on tap I would take Dark Arts because I know that to be better on tap. In bottle they are on a par and I can't decide between them. And don't even ask me about cask, the only time they are ever available on cask at the same time is at beer festivals. Maybe I will try a little side by side at the Winter Ales and Cask Festival in a couple of weeks. This is what I thought of it last year.
For readers outside of Ireland, O'Hara's is quite easily available* in many countries. Even on cask as far away as the USA. Dark Arts might be harder to get as it's a much smaller brewery and bottling is a recent development for them.
*as far as Irish craft beer availability abroad goes.