Samuel McIlhaga Johnston

I have twice had occasion to refer to a young family living in Belfast when they filled up the 1911 Census form. The clan link is the fact that they gave one of their sons the second name McIlhaga. Although on 15th May and 9th November last year I wasn't able to come to any positive conclusion, surely the fact that they spelled the clan name with one 'g' must be the clue to place them in a family tree. They lived at 33 Jersey Street, Shankill and were William (aged 31 so born 1880), Sarah (aged 26 so born 1885) Johnston and their children Robert 7, Samuel 6 and Evelyn 4. William was a Painter. They were all born in Belfast and William and Sarah were married (they said) in 1902. Unfortunately I have failed to find their marriage in the Irish records which would of course have given us Sarah's father's name. However, having attempted to reconstruct a McIlhaga family tree, I believe Sarah's father was named Samuel, and that this was why they gave their son Samuel the second name McIlhaga. I have concluded that Sarah was the second of the three daughters of Samuel McIlhaga who married Grace Marrs. Sarah's siblings were Margaret, born 16th October 1870 and William John Marrs, born 26th August 1877 in Belfast who married Norah Wellwood and had three children. Sarah's third and youngest sibling was Isobel who acted as a witness at the marriage of her brother. On 15th May last year, and on 6th July this year, I also referred to Evelyn V. McIlhagga Milligan who died in 1999. Could she conceivably have been William and Sarah's youngest daughter Evelyn, born in 1907? I have no evidence to demonstrate this except that I know of no other Evelyn associated with our clan.