Thomas Mary Nee Maher Irwin Ennis Killen Ireland to Australia

I am looking for information on Thomas IRWIN, born 1817 in Ennis Killian, Ireland, died 1901 and his wife Mary (nee Maher) born in Galway approx 1826 and died 1898. What I particularly want to know is how, when and why they came to Australia. They married in Yass, N.S.W in 1842. Any information would be much appreciated - K