He was also a cowardly assassin, but js makes a much better point.

As to your three points,

(1) If a country fails to act in the face of a perceived strategic imperative, it is likely to be destroyed. But if one country behaves in a way that its larger neighbour disapproves of, that is no justification for invasion (unless that "misbehaviour" constitutes a threat).

In the present case, when has Ireland been invaded for displeasing England by its behaviour? The Tudors? They conquered Ireland to quell a usurpation of the Irish Crown by an Irish Earl (threat), and they restored power to the Irish Parliament which had been assumed by the more powerful Earls and Chiefs. The Plantation of Ulster? Northern Ireland was settled by Scottish and English farmers who were given land taken from the rebellious O'Neils and O'Donnells (threat) in the hope that the new demography would be more amenable to English authority. Cromwell? The Irish were supporting the Royalist cause (threat - ironic that the Irish hate the enemy of England's monarch, whom their ancestors supported and fought and died for)

(2) The presumption being that England has no right "to be there". By that argument, virtually the whole population of the Americas - north and south - have even less right to be there, because they arrived much later. The same goes for Australia and New Zealand. Presumably, if the Aboriginals or Maoris started to foment revolution, you would say that the governments there could not complain about the cost in money or lives spent in maintaining order?

(3) Moral right ... England was invited, and being in control, it was obliged to maintain order; divine right (a) Pope Adrian granted the English Crown suzerainity over Ireland, and (b) the Irish Jacobites upheld the Stuart claim to the "Divine Right of Kings". Michael Collins, for example, was born a British subject, studied in King's College and began his career in London working for important financial institutions. His act of rebelling against the established government cannot be anything other than treason, and using violence against the state invites a violent response, even today in any country you care to name.