Thanks all for the comments, I'm glad for the discussion.
ndnhorseman, there are 2 important aspects of "White-Guilt" in America, in my view/experience:
1) While the English did indeed persecute the Irish for many many years, Southern Ireland eventually was given to the "native" people. It's similar to other past racial genocides (such as Germany with the Jews... now we have modern Israel). In America however, the First Nations have been displaced and dishonored, and their suffering continues to this day. I do not say this to victimize the people, only to highlight the fact that people like the Lakotah have been immensely betrayed, and given a seriously raw deal.
Anyone not totally blinded by their own ego or malice can see that... and so knowing, the closeness of the injustice can go straight to the heart of any reasonably empathic person. Guilt is the first response to that knowledge.
2) Interestingly, a great many whites in America have no spiritual/cultural center, because as ndnhorseman pointed out, they all migrated from their ancestral homelands in Europe. Since the 1960's especially, a certain portion of whites awakened to the idea of oneness with the land, which is an idea that underpins all First-Nations culture and spirituality.
So you have a situation where many whites (you might say the "hippies" and their offspring) became awakened to some aspect of the Great Mystery, and thirsted for it, and learned of the wisdom of the First Nations people. At the same time, they learned of the great injustices of their own forefathers who were ignorant of the wisdom and truth of the First Nations people...
It is like someone traveling across a barren spiritual desert, filled with concrete and asphalt, and stumbling across a beautiful Oasis (the ways of the Lakotah)... only to find that the waters are drenched with the blood of those you honor and respect, whom you sought to celebrate. Horror, guilt, shame, I believe all stem from that initial experience.
I feel this is the most significant factor which creates "white-guilt". It is difficult if not impossible for most whites to face that horror and shame, let alone to look about in the present and see more crisis for the First Nations. Sometimes its too much to bear, but bear it we must, and look instead to the tenacity and perseverance of the living Lakotah.
Where once I felt guilt, I now feel a sense of hope and inspiration. And if anyone should be pitied, or looked at as a victim, I'd say we should look to most whites in America who eat lies like candy, with very little hope of feeling awe at the great mysteries of life.