According to The Oxford English Dictionary, the earliest recorded use of 'left-footer' to mean a Catholic dates back only to the mid 1940s and that was in a military context. It could, therefore, easily be that it was only soldierly slang and had nothing whatever to do with either farm labourers or peat-diggers - or even footballers, John!
When military personnel are involved in a religious 'parade' situation, they are separated according to religion/sect etc and marched off to an appropriate building. Perhaps C of E and others - ODs (Other Denominations), as they were called - people were thought of by drill-sergeants as 'right-footers' and RCs as 'left-footers'.
I can't help thinking that - if it really had anything to do with Irish diggers - we'd have found an earlier reference somewhere in print, especially Irish print...Joyce, O'Casey, Shaw?
Over to Balderdash & Piffle!