Well! After years of trying to unearth the details I've finally found the parish birth record for my paternal great great grandfather James Byrne, born 20th August 1828 in Myshall, Co. Carlow Ireland (not Wicklow as I'd previously assumed). The next shock was to find he had been sent to prison for a month in 1848 for 'Hiding in a Haycock' (the precursor to a haystack). Why that was imprisonable isn't clear but the chap sentenced before him was stealing potatoes (this being the great famine)...