A long search for Dennis McEvoy's origins

Dennis (Denis) McEvoy, an agricultural labourer aged 28 years, came out to Australia from Blackrock in Dublin, Ireland, arriving at Pt Adelaide on 17th June, 1840 aboard theCharles Kerr, accompanied by his sister Catherine McEvoy, a dairymaid aged 20. Rumour has it that another sister was set to accompany Dennis but changed her mind and Catherine took her place. It is thought they were half English on their mother's side and half Irish. The Charles Kerr  left London on March 6th, 1840 

Catherine McEvoy,  born 1820 married another Irish immigrant, Thomas Henry, son of John Henry and Ann Murphy born in County Meath, Ireland in 1812, on 6-7-1841, only a year after she arrived in Australia, and in March, 1842, moved to New Zealand. The Henry family bible states that Catherine and Dennis were the children of Patrick McEvoy and Mary Bowes (best guess-this surname is almost unreadable) of Queen's County, Ireland. 

We know they had a sister, Margaret, married to Bryan Kilbride, who lived in Athy, County Kildare in 1858. We have been able to trace some of her family. We also think that  Dennis, Catherine and Margaret had sisters called Mary and Anne and possibly a brother G.......   The family story is that Dennis McEvoy was the 7th son of a 7th son, but if that is so, we don't know about any of the brothers or where they lived. 

Dennis died as the result of a cart accident in 1865 and was said to be 60 at that time which would mean that he was born in 1805 and also that he was 35 years rather than 28 when he came to South Australia. He possibly lied about his age because you had to be 30 years or under to get free passage to South Australia at the time. This may also be the reason why so  little is known about their origins in Ireland. I don't think they actually lived at Black Rock but just waited there to catch the boat to Australia.

I would be delighted if anyone could help me with information about the parents, brothers and sisters of Dennis and Catherine  and tell me where they came from in Ireland. It is just like they didn't exist until they arrived in Australia. I have been working on the family history for years now-see  http://www.louisenordestgaard.com/McEvoys-SA.html