The answer to the question is fairly simple. The song makes references to "the Auxies" and "the Black and Tans" so that puts it in the period after 1921 and the end of the Anglo-Irish War. What followed was a civil war in Ireland between the "Free Staters" [The Government of Ireland at the time] and the IRA. When it became obvious that the Government was going to win this civil war, a campaign of wanton destruction was embarked upon by the IRA that seriously hampered the ability of the economy of the Irish Free State to grow. One of those who actively encouraged the IRA in their fight was one Eamon de Valera and later when he came to power in Ireland, his attitude towards the British and trade with the British held the country back even further. Net result of this Dick, was that there was no work to be had in Ireland, so the labour force went to where the work was.