Apart from the information on John and Isabel's marriage certificate, on the birth certificates of their children and on the marriage certificate of another of Isabel's children, Victoria (this suggests John may have been a contractor in 1884), we know very little about John Roberts and in view of other inconsistencies even that information cannot be relied upon. He was never with Isabel on any census. If the John Roberts, aged 24, living in Llanidloes in 1851 was the man Isabel married then he might have been the John Roberts, aged thirty-six, who sailed from Liverpool aboard the ‘Emerald Isle’ and arrived in New York on 18 May, 1863 but I have not found John on the US censuses. In 1869, John and Isabel's eldest daughter sailed to the United States and the following year she married, in Philadelphia, Robert L. Jones, a native of Penmachno in Caernarvonshire (see here).
It could be that John Roberts had known Robert L. Jones in Wales; they may even have been distantly related. Isabel and John's marriage certificate states that John's father was also called John and that he was a farmer. There was a married couple, John and Jane Roberts, living in Penmachno, the village where Robert was born and grew up. They had four children, all, according to the IGI, christened at Penmachno: Robert (ch. 20 Aug. 1820): Mary (ch. 23 Dec. 1821): Jane (ch. 27 July 1823) and John Roberts (ch. 14 Aug. 1825). In the 1841 census John Roberts (54, a farmer), Jane (49), Mary (18) and John (15), together with an Ann Roberts (9), all natives of Carnarvonshire, were living at Penbedw, Penmachno. I believe the Penbedw farm was about a mile east of the Penmachno quarry, which was at Cwm Penmachno, and about 1.5 miles SSW of Penmachno village; the whole area was called Penmachno. In the 1851 census the parents, John (67, a farmer of 50 acres, born in Penmachno) and Jane (62, born in neighbouring Yspyty) - inconsistencies in age are commonplace in old censuses - were at the same address with their other two children; Robert (29, a farm servant) and Jane (25, a house servant). The 1841 census contains little information but the 1851 census states that both Robert and Jane were born at Penmachno. On the basis of the scant information available about John Roberts, this may well be have been his family.
Further, in 1841 there was a Jane Roberts (16) living alone as a farm servant at Llanystyndwy (now called Llanystumdwy), which is less than 4 miles from Llanarmon, Caernarvonshire, and about 25 miles WSW of Penmachno. This may have been the above-mentioned Jane. In 1851 the John Roberts recorded at Llanidloes was reportedly born in Llanarmon, Flintshire; perhaps the informant actually meant the Llanarmon in Caernarvonshire. Early censuses often contain half-truths, particularly if a friend or neighbour supplied the 'facts'.