I'm just not getting the 'historic' aspect here. Mr Higgins has visited the UK several times since becoming President. This is just a normal state visit the same as for any other country. This one's just a bit fancier than his previous visits.
The really historic one was when the Queen went the other way, particularly given the very high threat to her life while in the Irish Republic.
Before anyone complains, I was born and raised in Northern Ireland, although I've lived away from the province for going on 6 years now. I know the issues between the Irish and the British as well as anyone.
On a lighter note, I didn't think there was a shorter head of state than the Queen, but apparently I was wrong!