I'm sure he's not the one buried at Shortland Cemetery, Thames. No original documents list 'Swanton', nor has the name been handed down as a middle name. He was Ryecroft at marriage, and variously Rycroft and Roycroft for the children's Howick baptisms.
There was a Roycroft/Swanton marriage in West Cork, Ireland, but much later than our Robert who was born in 1798 in Carlow.
Any connection with a Robert Roycroft and Elizabeth Sibery/Siberry/Sibere also appears incorrect, as they were busy having their family in Rutherglen, near Glasgow in the 1800s, not in the 1700s, as contained in some New Zealand Roycroft trees.
There's a possibility that Robert (1798) Roycroft was descended from some Roycrofts in Roscommon, Leitrim, which I'm following up. The main information we know is that our ancestor was born in 1798 in the middle of the United Irishmen's Rebellion, with battles / troops all round Carlow. His Dad was very likely a Fencible too, as the regular British army was tied up with Napoleon at the time, and detachments came from other provinces to put down the rebellion. Also, he may not have known his father, as the sons all appear to have Jane's McIntosh family names.
A check on 'Carlow 1798' in a search engine gives the clues. Joan