Hi Andrea,
You seem to have most of the info. re.David Armstrong
He was born 10th August 1876 at Brackenhill near Glenavy.
His birth was registered two days later on the 12th August by Elizabeth Dawson who was present at his birth.
His father was also David and his mother was Eliza ( formerly Judge)
As you know he married Elizabeth Larkin from nearby Aghagallon and they then moved to Belfast were they had five children.
Mary,John,Edward,Joseph and last Alice who died tragically at 14.
David had an older brother Joe who lived in the McDonnell Street off Albert Street in Belfast. Two of his sisters lived in the St.James Road area and played music in the Grand Central Hotel in the city.
I hope your husband has good luck in Glenavy and just a tip, tell him to check all the Glenavy churches as the Armstrongs straddled all denominations through the years,. Best Regards