If you search the online 1901 County Sligo census just for the O'Hara surname, you'll see there are 151 entries for the surname in that census. That's after the Famine (1845-50) and substantial emigration from 1850-1900.http://www.leitrim-roscommon.com/1901census/census.shtml
Also if you search Griffiths Valuation Index Extracts for the O'Hara surname in County Sligo 1858 there are 200-300 entries for occupiers of lands named O'Hara.http://www.failteromhat.com/griffiths.php
O'Hara is an extremely name in surviving records in County Sligo. The same surname doesn't mean the same family or same family line.
Major General Charles O'Hara was a major estate owner in County Sligo with the Annaghmore Estate. See:http://www.proni.gov.uk/records/private/coophill.htm
The 1876 Landowners of Ireland notes he owned 21,000 acres of land in County Sligo. See:http://www.failteromhat.com/lo1876/322.pdf
With such a large amount of land, those with surnames O'Hara (common in County Sligo) would have occupied lands which he owned.
You can search the Kilvarnet & Ballysadare, Sligo Roman Catholic parish records microfilmed and available for research on order through your nearest LDS Family History Centre for more information about your Denis O'Hara and his family:http://www.ireland.com/ancestor/fuses/rcparishmaps/index.cfm...
And here's a link to the Roman Catholic parish maps in County Sligo, click on any parish on the map to see the detail years covered in the records and where they might be accessed:http://www.ireland.com/ancestor/browse/counties/rcmaps/sligo...