'Legal' is actually one of those very interesting words. What makes something legal?
Because the lawmakers say so. What makes the lawmakers legal? Because they say so.
Was the Norman invasion of Ireland legal? And if so, what made it so
Debatable. The Norman "invasion" was by invitation, but the person who issued the invitation was a dispossessed king.
Well, an English pope declared it legal for Henry the Second to invade Ireland, to convert it from the Celtic church to Roman catholic. Does that make it legal?
Except Henry II didn't bother: he dropped the idea completely.
The other kings, and queen Elisabeth, simply wanted it. They were the (legal?) rulers but not over Ireland, so what made it legal? We stole it ourselves, so now it is ours?
The thing about kings and queens is that they often reign over more than one kingdom at the same time. Here, English monarchs were also Lords or Kings/Queen of Ireland: they did not want Ireland - they already had it. They didn't steal Ireland, they ruled it with an independent Parliament (initially and latterly) with the consent of the aristocracy and the clans ... except when individual aristocrats and chiefs fell out of favour, they rebelled in the name of the Irish.
America - was already occupied. Was it legal for the settlers to take it from the Indians? Was it legal for the Britsh to simply declare it theirs? With what right?
Was it illegal to settle America?
How long does it take for something to have lasted long enough to make it legal, by force of habit?
Or is it legal by consensus?
By force and arms?
All of those things. Here are a few dates:
1241 - English Parliament formed
1297 - Irish Parliament formed
1788 - Federal government of USA founded
1849 - Danish Rigsdagen formed
1937 - Oireachtas of the Republic of Ireland formed
1953 - Danish Folketing established (out of Rigsdagen)
I don't think there's any doubt as to the legitimacy of Danish laws, is there?
Thomas Jefferson said, "Governments are instituted among Men, deriving their just Powers from the Consent of the Governed." I see no reason to disagree (although I deny that withdrawal of consent justifies rebellion).
Law and order was what they had, more so than many countries. Violence and disorder is what you get when your force yourself on another culture and change it with violence.
For reasons already given, I disagree that the Irish brehon laws made Ireland a civilised country: Early Irish Law concerned itself with how a man might improve his status, and how much he might have to pay for killing or wounding someone - which varied according to the status of the victim. I believe the early Danes had similar systems before they were Christianised.
Brehon law had no judges or courts: it was pronounced by the local chief or wise man who would interpret it as he saw fit; and it would be enforced by the people affected - the mob.
The Irish were always violent. Clan feuds were routine and frequently bloody. When not fighting among themselves, the early Irish would raid South West England (cf Tristan & Iseult), Wales (whence came St Patrick), Isle of Man and Scotland (or Pictland - which they colonised by creating Dal Riada and progressively eliminated or absorbed the native Picts).
I don't think anyone has shown me where England "forced" itself on the Irish