Irish couple were found dead in the downstairs of the property by police before sons Liam, 13, Niall, 11 and six-year-old Ryan were discovered upstairs
The Irish Mirror reports Alan Hawe left the note with instructions that it should only be opened by relatives.
He also left a note outside the property in Ballyjamesduff, Co Cavan, Ireland, warning visitors not to come inside.
Alan Hawe and wife Clodagh were found dead in the downstairs of the property by police before sons Liam, 13, Niall, 11 and six-year-old Ryan were discovered upstairs yesterday
One of Mr Hawe's devastated friends said the dad of three was meticulous in everything he did.
PA
And he said he had been devoted to his wife and children throughout their 15 year marriage.
He said: "Alan never put a step wrong. What he started, he finished.
"If he said he would support you, he did. He was a man of his word and a good man and that's why this is all so confusing.
"I feel so bad for Clodagh and the boys, but for Alan too. What was he going through?
Philip Fitzpatrick
"It's impossible to understand but we will understand and the truth will come out, I feel sure of that. I've been told Alan made sure of that in some detail.
"For the man that Alan was could have left no loose ends.
"Something broke in him but even so, he would not have just walked out of this life without an explanation, without letting people know why.
"I don't know what or how he did this or why he thought it was the only way out for him and the family because he loved them all with all his heart.
"But he left his reasons for the people he loved, he explained, I know that much. And that would be typical of Alan."
The friend said he had seen no clues to the desperate state of Alan Hawes' mind in the days leading up to the murder suicide in the family home Co Cavan.
He said life had appeared to be perfectly normal before the disaster was revealed to the world in a note pinned to the back door.
It is unclear yet whether the family will be buried togeth