Feeney is described as a Connaught Ranger who served in the Afghan War and then deserted in Calcutta before making his way home. However, the 88th Regiment of Foot (Connaught Rangers) were based in Malta during the Anglo-Afghan War and went to the West Indies in 1847. The Anglo-Afghan War was fought by soldiers of the British East India Company, not by the regular British Army.
Early on in the film Feeney's sister-in-law, Ellie, returns from foraging for food. She tells her daughter that she had only been able to find nettles. The story takes place in winter so nettles, being dormant at that time of year, would have been unavailable.
The scene with the train could never have happened in 1847 as the first train to Galway was in 1851 and it was on a single line, not a double track. The train featured is also far too modern and large in steam terms and pulled too many wagons. It did not go to Connemara at this time nor was it standard gauge.
The pub pictured at the end of the film has windows made out of wire mesh glass; this was not invented until 1892.