I also found my great grandfather in the same barracks in 1901. Both men were the same age and may have even gone to school together. (I can check the school records next week) I vhecked the 1911 census but your Grandfather is living at 61 Unity Street, which used to be in Belfast...not Ava Street, Lisburn?
From Hillhall rd to Ava st. is a short walk. See below.