Tom
If Robert's mother was Margaret McIntyre, then these are the most obvious entries on 1841/51:
1841:John McIntyre 50, lab., b. Ireland
Jean McIntyre 40, b. Ireland
Margret McIntyre 20, Muslin Flowrer
Charles McIntyre 17
James McIntyre 15
Elisabeth McIntyre 12
Joseph McIntyre 9
David McIntyre 6
Uphemia McIntyre 4
Ann McIntyre 2
Robt McIntyre 1
Address: Town Head Street, Stevenston, Ayrshire
1851:As transcribed. Maybe a second marriage for John Snr (or wife's name mistranscribed in one of the entries). Everyone apart from John and Elizabeth showing as born in Stenster (Stevenston most likely
)
John McSutyre 61, coal miner, b. Ireland
Elisabeth McSutyre 53, b. Ireland
Margaret McSutyre 30, Hand Needle Sewer
Charles McSutyre 27, coal miner
James McSutyre 25, coal miner
Joseph McSutyre 19, coal miner
David McSutyre 17, coal miner
Euphemia McSutyre 13
Ann McSutyre 11
Robert McSutyre 11John McSutyre 8
Hugh McSutyre 5
Jane McSutyre 3
Address: 6 Cressent Row, Eglenton Iron Works, Kilwinning
With the family in Kilwinning, this would leave Margaret well placed for her marriage to Peter McCartney in Kilwinning a couple of months later.
This looks like Margaret and Peter in 1861:
Peter McCartney 37, Silk Weaver, b. Kilwinning
Margaret McCartney 38, Muslin Sewer, b. Stevenston
Marion McCartney 7, b. Stevenston
Peter McCartney 3, b. Stevenston
Address: 21 Schoolwell St, Stevenston
Margaret's death cert would hopefully let you confirm you have the right family in 1841/51:
1873 MCCARTNEY MARGARET MCINTYRE 54 STEVENSTON /AYR 615/00 0072
Tom, it is not uncommon for illegitimate children to use the surname of their reputed father if known to them in later life (and use them interchangeably which can be confusing). From the way Robert used the surname FREW, it would appear that he knew the name of his father and used it in later years. Not sure if you will necessarily know who his father was though unfortunately.
Monica