I believe we are relatives. My mother's family are Leonards from Mountcharles Donegal, formally Lenaghan. They still live there today. I heard a similar story of three brothers moving to Donegal but I heard from Cork not Sligo. This is information I have been seeking to have confirmed.
Either way I believe the Hugh Lenaghan you are refering to lived in Drumgorman. Like you said his father was also Hugh. He was one of the three brothers that moved to Donegal. I have info on Mary moving to Brooklyn and marrying John Connaghan from a cousin of my mothers.
My grandfather's best friend was his cousin, John Lenaghan, who would be your father's brother. He emigrated to England as well.
I have much more info than this. I would be very excited to exchange whatever information we both have.