Ben, Dublin is like a separate country from the rest of Ireland so it doesn't surprise me that you were given that information.
If the Family History Centre you used in Dublin was the Genealogy Service at the National Library or National Archives, that was staffed by members of a professional genealogy organisation whose members all live in Dublin and who were employed to staff the genealogy services on a contract basis. Many of them would have no or very limited knowledge of places or records available at either archival source for Westmeath or other locations outside Dublin which they'd consider "down the country". However, that organisation's contract has not been renewed so hopefully each of those archival locations will get staff more familiar with places elsewhere in the country or the available records to find information about them.
If you went to the General Register Office, staff there aren't genealogists at all, and most of them have no genealogical research experience and wouldn't have to in order to provide the civil registrations records there for people to search since all the records are in date order for each of the bmds.
Many people in that area of Westmeath and across the border in County Offaly have family connections because families went back and forth for employment, housing or socialisation. The county border wouldn't make any difference or be a barrier for the family connections.