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The 1911 census shows the family living in Lisburn, Co. Antrim and contains lots of details to help you find more information.
Robert Heathwood is Church of Ireland, age 57 and born in Belfast. Therefore, since he was born c1854 there will be no birth certificate. Any baptisms for his children are likely to be in Church of Ireland but you'll notice from birthplaces in census that the family moved around a bit (probably to do with his work).
The census says wife was Letitia Johnson- notice that Heathwood's been rubbed out and her surnae changed to Johnson. Perhaps Robert thought he was supposed to list his wife under her maiden name. Married 27 years so you need to look for a marriage c1884 possibly in Antrim town as that's given as her birthplace.
Eldest 2 children (Lettitia, 22 and Dora, 20) born Belfast and next three (Emily, 18, Thomas, 13 and Maggie Ann, 10) born Carrickfergus.
Two grandsons also listed. John Bell, 1 probably belongs to a married daughter named Bell who isn't in the household in 1911 but could be living with her husband elsewhere. Other grandson Thomas James Heathwood (7 months) could belong to a married son or unmarried daughter.
Will post some information shortly.