Why is the Queen's state visit to Ireland today (17/05/2011) being touted as 'historic', and why is security so tight surrounding her trip?

The Queen has of course visited NORTHERN Ireland many times in the past, but she hasn't been to SOUTHERN Ireland (Eire) before, therefore this will be a first for her.
Whether this is a good idea, I am not sure; because of the historical, political, and religious sectarianist grudges, many will unfortunately not be happy to see her.

If you want to find out the reason and conflicts in Ireland look up history of Ireland 1600-now on politics but in general there is a massive divide and the queen is a symbol of rebellion in ROI but I remember roads being blocked off and checked everyday before the queen came and this waz all for 1 person and the money spent annoyed many people aswel

She has been there before but not the southern part.

It's historic because she's only the second Queen to visit southern Ireland, I suppose. But she has no welcome here. We are not happy to see the British Queen being pampered with 30 MILLION EUROS spent on security while Ireland's own people are rotting in the ditches.