Then you'll need to contact the Church of Ireland Archives in Dublin to find where the church registers might be accessed that might have baptismal or marriage records for the family. It would be very helpful for you in your research if you had a bit more information than just the surname and Newtown. For example, the given names, parents names, dates of birth, etc.
Explain you have a location for the family in Newtown, County Westmeath and you understand that's a Civil Parish as well as a townland in the Civil Parishes of Ballymore, Mayne, Mullingar and Castletowndelvin and you want to search for family information in the church register. Most likely the reference was to Newtown Civil Parish because the Civil Parishes were generally the same as the Church of Ireland parishes. But of course, it's always possible the reference was to the townland.
Here's the contact information for the Church Archives. They're generally very prompt responding to queries but there won't be anyone there on the weekend and Monday is a holiday so you won't get a response until later next week. http://www.ireland.anglican.org/library/libroots.html