The Herald has learned that in a long and rambling note left on the kitchen table of his Ballydehob home in west Cork, the 50-year-old staunchly defended his intended actions.
In the highly emotional letter to his devastated wife Rebecca, the farmer insisted that he had no other choice but to take the tragic course.
"Unusually, it was also exceptionally long and it's likely it took at least an hour to compose," said a source.
Mr McCarthy detailed his irrational fears that his marriage was at breaking point and that he couldn't bear to be separated from his daughter.
"He was worried that if the marriage ended he would see less of Clarissa. He said that he couldn't bear that and it was all too much," explained a source.
Gardai now believe that the farmer had carefully planned killing his only child and then himself, arranging to have time alone with his daughter on Tuesday evening while his wife visited a friend in the west Cork village.
Mr McCarthy was said by neighbours to have been "absolutely devoted" to his little girl and very distraught at the potential break-up of his relationship.
The note now forms a crucial part of the garda investigation into the double tragedy.
The alarm was raised shortly after 11pm when Mr McCarthy's American-born wife, Rebecca Bailey-Cejnar (26), discovered the note he had left at their home at Foilnamuc – some 5km outside Ballydehob.
She had called to the house after becoming concerned about the whereabouts of Mr McCarthy and her little girl.
The woman was so shocked by the note's contents she immediately alerted gardai and a major search operation was launched with the aid of the Irish Coastguard and RNLI.
Rebecca is to be supported by family members who are flying from Los Angeles to be by her side.
The body of the little girl was found at 1.30am yesterday on the strand at Audley's Cove, just 1km from the family home.
The body of Mr McCarthy was found in shallow water at 2.30am over 200m away near an offshore island. Both were pronounced dead at the scene. They died sometime between 8pm and midnight.
Preliminary tests indicated that both died from drowning in salt water.
Locals said that the couple, who married in 2006, had seen their relationship run into difficulty over recent months. The American woman was just 19 when she married Mr McCarthy who was 23 years her senior.
They had met while she was working as an au pair in west Cork. The couple's only child, Clarissa, was born in 2006. Plans were already underway for her fourth birthday party in April.
Gardai described it as "a tragic incident" and said the nature of their investigation would be determined by the results of the post mortem.
However, garda sources said that they are not looking for anyone else in connection with the matter.
The distraught mother was last night being comforted by west Cork friends with members of her US-based family now flying to Ireland.
The farmer, who also worked part-time for a local vet, was a lifelong Fine Gael activist and canvassed for the party in the Cork South West constituency.
Former Cork TD PJ Sheehan (FG), a good friend of Mr McCarthy, said he was "deeply shocked" by the tragedy.
"My wife Frances and myself were at their wedding. She (Rebecca) is a lovely woman with a great personality. I could say he was tremendously happy with married life.
"Martin was a man you could rely on. He was a professional farmer who was energetic and highly respected. He was absolutely devoted to his wife and child and the little girl was his pride and joy."
The farmer had been in poor health recently and had been treated in CUH for a heart complaint before Christmas.
Several locals said that, while a shy man, he had been unusually subdued over recent weeks.
Mr McCarthy's former schoolmate, Leslie Swanton, said the entire Ballydehob community was devastated.
"He was an absolute gentleman. I knew Martin all my life. We went to school together for a spell. I am totally shocked."
Rebecca's friend, Catherine Norman, fought back the tears as she urged people at the scene not to rush to judgment over the tragedy.
"No one can say... no one can judge. This is a terrible tragedy not just for the family but for the entire community," she said.
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