Until the English managed to conquer Ireland the country was fractured into various small kingdoms and fiefdoms. What were the reasons that no one was able to unify them into a single kingdom? Was it mainly because of outside influence (i.e. the English) that prevented this from happening? And also what were the factors that allowed Scotland and England to unify into their respective kingdoms?
As a side question almost how close did Brian Boru come to creating a single kingdom of Ireland? And had he and his sons not been killed at Clontarf would it have lasted?
Ireland ran out of time before Henry II felt the need to invade in 1171.
England had no single King until Athelstan (d 939). Malcolm I (d 954) became Scotland's first single King when he conquered Strathclyde. So Brian (d 1014) was not that late relatively speaking.
Brian came close to a single Irish Kingdom; the Vikings and other provincial kings acknowledged his overlordship as High King. In fact, you could easily argue that Ireland was not ruled as a single Kingdom until 1603 when Elizabeth I finally defeated the old Gaelic lords.
It is possible that if King of Leinster, Dermot MacMurrough (d 1171), had lived, he or his son-in-law Strongbow would have defeated the then current High King (O'Connor) and properly unified the Kingdom with superior Norman arms and a change of succession from tanistry to a hereditary just as Malcolm II of Scotland had done. After Strongbow's ally died, Henry II invaded and an independent Hiberno-Norman Kingdom never formed.