Sometimes it's the landscape that gets me as much as the monuments, the Burren in Co. Clare in spring, wild Donegal in winter, autumn in the Sperrins of Tyrone.
For me, I think the monuments condense not just the memories of the site but also the journey there and the landscapes wandered, into a single package that makes me want to visit at other times of the year, the day or the night. The changes of the seasons and the hours of the day mean less and less in modern life but they do make a huge difference to the experience of 'a visit' and tend to keep me in touch with whats real and whats 'fake-real'.