The births of the children you mention all seem to have been in Skerry townland where Hugh Hilferty/Helferty was a farmer.
A John Helferty is listed in Griffiths Valuation of 1858 as having plot 2 in Skerry. That was a farm of just under 9 acres. There were about 10 houses in the townland then and no other Helferty households. So that looks strongly to be your ancestors’ home. I believe John Helferty was Hugh senior’s father. That property today is near Lake Keel a mile or two north of Kilmacrenan. There were McCafferty households in the townland so it’s possible Hugh married the girl next door.
There are 2 possible deaths for John Hegarty. One in 1877 aged 70 and the other in 1876 aged 60. (Both registered in Millford). Neither is on-line yet but you can order copies from GRO Roscommon for €4 (euros), to see if either is your family.
John’s wife Catherine died at Skerry in 1889 aged 90:
You will notice that the informant was her daughter in law Ellen. So that pretty well proves John Helferty was Hugh senior’s father.
I spotted this marriage in 1868 between Cecilly Helferty and Patrick McDevitt. I don’t see them in the 1901 census so suspect they left Ireland. Her father was John. So she looks to be Hugh senior’s sister.
I found children to Hugh & Ellen as follows, all born at Skerry (farmers tended to stay in the one place):
Civil registration of marriages started in 1864. I don’t see a marriage for Hugh & Ellen there so suspect they married before 1864 (in which case Hugh was probably born a bit before 1851). Unfortunately the RC parish of Kilmacrennan doesn’t have any marriage records before 1864 so there’s probably no trace of the marriage. (There are some trees on Ancestry which have the couple marrying in Dunfanaghy district in 1865 but I don’t see a marriage in the civil records that accurately matches that information. I suspect it’s wrong).
Again some trees on Ancestry have Hugh Hilferty dying in Donegal in 1893. I can’t see a statutory death that confirms that though. His widow appears to have moved to Philadelphia in about 1894. There were no Hilferty/Helferty households in Skerry by the 1901 census: