Since the marriage may be sometime after 1900 your best bet would be to search the civil index. (Most of the online parish records dont include more recent dates)
When searching for Mc surnames on familysearch you need to try the name in the formats M'Carthy (that's a single quote), Mc Carthy and just Carthy (the Mc could be dropped). You may also need to allow a few years either way for year of birth if you are basing this on age at death - this was not always accurate as it depended on the details known by the informant.
Shane