Most of the animosity is bred from historical actions. Old scars cut deep, but also, in these days of mutual help and economic solidarity, they are (frankly) stupid.
England has been the leading nation in one of the planet's most influential empires and to acquire other nations in that manner you have to step on toes. The British Empire did many bad things, but it also modernised and 'civilised' as it went. People prefer to forget the benefits of British rule in order to be able to blame someone else for past injustices. Add in the fact that 'empire' has become a very dirty word and the fact that the BE was the most recent biggun and England becomes an easy target.
Of course, Irish and Scortish hatred usually stems from past attempts at conquest, the handling of the Great Famine, the Highland Clearances and so on. But dig into the historical record and you find that both Irish and Scottish landowners ****** their own people over, too. But that doesn't quite fit into the nationalist's plucky peasant Braveheart POV so it is brushed under the carpet in exchange for an evil English shadow. Of course, for the ROI there is also the matter of The Troubles, but I haven't read much on that so can't comment.
I'm not denying that some English acts were terrible, but in mitigation, those centuries gone by we're extremely violent and bloody. With roles reversed the Scots et al would have done similar.
Personally, I'm like most Englishmen - part Irish, part Scottish. When you look at what our tiny countries achieved TOGETHER it is astounding.