Hi
So you have Timothy O'Dea born 1912; and some siblings, do you have the other's names? If born before him , then they and their parents, Michael O'Dea and Mary (Flynn) O'Dea should appear in the 1911 Census, (probably) at Galbally, co.Limerick. If you have other siblings names, the family should be fairly easy to identify - you say Timothy was born in Newtown, Galbally, so you have the townland and parish, which is good news.
Ages of Michael O'Dea and his wife : probably born sometime in the mid to late 1870's - although if it turned out Timothy had loads of siblings born before him in 1912, then the parents could be older than that.
As Kate said, if you have a Family HIstory Library near to where you live, you can go in and order the relevant microfilms for the census you require. There is a branch in Sunderland, for example.(I would think most branches will be on the verge of shutting down for xmas now).
Marriages usually took place in the bride's parish , but not always; good chance that the married couple took up residence in Mary Flynn's home parish.
Griffiths Valuation ( a nationwide survey which took place in early 1850's in Limerick) shows the name Flynn, and Dea (maybe O'Dea, not sure if same name), neither in Newtown, but these names would be of people old enough to own or lease land, so if connected , could be Timothy's grandparents or relatives to them :
http://www.failteromhat.com/griffiths/limerick/galbally.htmAnd finally, this site is good intro to how to go about tracing Irish ancestors:
http://scripts.ireland.com/ancestor/index.cfmJust a few thoughts - and good luck!
Cheers,
Mags