I can see a birth for Samuel registered in Kilkeel on 11th April 1877. Mother?s maiden name Colgan. Possible siblings:
William 26th Dec 1866 in Newry (mothers maiden name Colgan)
Robert 1st Sept 1868 in Kilkeel (mn Colgan)
Hannah 15th Oct 1869 Kilkeel (Colgan)
Sarah 11th April 1871 (Colgan)
The townlands and father?s occupations on those certificates may help trace the family in the revision records. Do you have that information?
There?s this man in the 1901 census:
http://www.census.nationalarchives.ie/pages/1901/Down/Victoria/Chadolly_Street/1217331/
He died in Belfast on 4th April 1907, est year of birth 1841. The Henry in the census was born in Downpatrick 16.6.1875. Mother was Ann Colligan. Might just be a coincidence. Ann Ard died 12.1.1904. Also in the same grave is a Martha Beattie 19.5.1896 aged 32. All are buried in the City Cemetery.