The best plan is to go out to the centre for opening, around half 9 if I remember correctly, and then pick what time you would like. You cant really pre-book, you have to go to the centre and buy tickets for whatever slots are available. Going out for opening time just guarantees you get a tour and lets you choose. I've gone a few times over the winter months in the morning, bought a ticket for the last tour in the evening so you are there near sunset. Unfortunately it closes far too early in summer. The place definitely changes when the light starts fading, the facade is less blatant and 'in your face', a totally different feel to the place than picture postcard blue sky daytime tourist trap.

Whatever you do, do Loughcrew as mentioned above! I might bump into you, I'll be in that area for the next few weeks getting some photos together for the exhibition.