Sandra, have you checked the Catholic records at the IFHF website http://ifhf.rootsireland.iehttp://ifhf.rootsireland.ie ? I did an all-Ireland advanced search there on counties Tipperary, Laois and Offaly combined and could not find any record for a baptism of a child born to a Thomas McNamara with a mother named Kavanagh. Are you sure those are the names of the parents? Do I understand correctly that Thomas McNamara and Catherine Kavanagh had two daughters, named Mary McNamara (b. 1848) and Catherine McNamara(b. 1845)? Most parish registers in the area would have begun by then, but perhaps some have gaps.
Roscrea is the name of a town in northern Co. Tipperary, but it is also the name of (1) a civil parish and (2) a Catholic parish in the same area. Both the civil and the Catholic parish of Roscrea have parts in two counties - Tipperary and Kings (now called Offaly). During some time periods, part of the Catholic parish of Roscrea also extended into Queens County (now called Laois) and was known as Roscrea & Kyle.
It is therefore very possible that your McNamara family lived in the parish of Roscrea (either the civil or the Catholic parish or perhaps both) but in Kings County rather than in Tipperary.
Catholic parish boundaries changed over time as parts of one parish were split off and joined to parts of other parishes to make a new parish. It is often difficult to figure out which areas were included in a particular Catholic parish at a particular time without examining the parish registers.
Ifsomeone left a record with a birthplace listed as Roscrea, did that mean the town (or near the town), the Catholic parish, or the civil parish? It is hard for a researcher to know which one was meant. That is one of the problems with doing Irish research: all these different administrative divisions might have the same name (Roscrea in this case). At least in your case, the county was mentioned as Kings, so you know it was probably not the town of Roscrea proper that was meant, but the parish (civil or Catholic) of Roscrea, or perhaps somewhere close to the town of Roscrea but in Kings County.