I think I might have figured this out after a lot of scrolling and head scratching.
The census records already posted suggest that these are the siblings who are the children of William - ages are quite fluid in all the available records.
Johnborn 1864/1869
Patrickborn 1871
Juliaborn 1871/1872
Thomasborn 1871/1874/1876
Maryborn 1878
Williamborn 1882/1883
Starting with Thomas because he is the subject of the enquiry, I think that this is his registered birth and baptism.
ThomasBaptised 23 Aug 1874
Son of William HEALOW and Anne SMYTHE
https://registers.nli.ie/registers/vtls000635215#page/35/mode/1upBorn 5 Sep 1874
Son of William HEALOW and Anne SMITH
of Randalstown, Kilbarry
https://civilrecords.irishgenealogy.ie/churchrecords/images/birth_returns/births_1874/03139/2151148.pdfI know that the dates do not match but I have seen many Irish births like this and maybe it was a case of not registering within the required timeframe. Perhaps the correct birthdate was 5 August.
Kilbarry/Kilberry, Meath matches Thomas' stated place of birth when he enlisted.
MargaretAlso baptised and registered at the same place was Margaret in 1871, parents William HEALY/HEALOW and Anne SMYTH/SMITH. Baptised the same day she was born so perhaps she wasn't expected to live.
https://registers.nli.ie/registers/vtls000635215#page/31/mode/1uphttps://civilrecords.irishgenealogy.ie/churchrecords/images/cert_amends/cert_1872/2196075b.pdfJuliaJulia was also baptised at Oristown on 17 Jan 1869, parents William HILO and Anne SMYTHE. The birth was registered as 'Judith' (which is a name interchangeable with Julia), parents William HEALOW and Anne SMITH. This time they are at Clynch, Kilbarry. Date of birth is slightly at odds again and she later thought she was born in 1872.
https://registers.nli.ie/registers/vtls000635215#page/28/mode/1uphttps://civilrecords.irishgenealogy.ie/churchrecords/images/birth_returns/births_1869/03411/2250995.pdfPatrickBirth registered at Moynalty, Kells in 1866, parents William HAYLOW and Anne SMITH of Newrath which is a townland in the parish of Kilbeg.
https://civilrecords.irishgenealogy.ie/churchrecords/images/birth_returns/births_1866/03544/2304516.pdfI think that this is the baptism of that child on 7 Jan 1866 at Kilbeg with mother's surname incorrectly recorded as REILLY. Father William HAYLO.
https://registers.nli.ie/registers/vtls000635216#page/63/mode/1upJohnAlso baptised at Kells on 10 May 1863, parents William HELOE and Anne SMYTH
https://registers.nli.ie/registers/vtls000635212#page/160/mode/1upMaryBorn after Thomas around 1878. A bit out of the area of the other children, a birth at Stamullen, Drogheda on 28 Jun 1877, parents William HALLOW and Anne SMITH of Snowtown.
https://civilrecords.irishgenealogy.ie/churchrecords/images/birth_returns/births_1877/02999/2099150.pdfBaptism at Ardcath on 29 June to William and Anne HALO.
https://registers.nli.ie/registers/vtls000635052#page/126/mode/1upWilliamThe youngest child William, born and baptised at Collon north of Drogheda in 1880. Parents William HEALY and Anne SMITH. Julia HEALY was one of the sponsors, perhaps his sister.
https://registers.nli.ie/registers/vtls000632531#page/68/mode/1uphttps://civilrecords.irishgenealogy.ie/churchrecords/images/birth_returns/births_1880/02862/2048839.pdfHaving a bit of a Google, the surname may have progressed from the original surname HEALAUGH.
Debra
