The Meri- in Merimac's name possibly comes from Old Welsh for 'great', but -mac is obscure. We do know that the -mac ending was used in actual Hobbit-names (as opposed to the more familiar 'modernised' versions) so it is possible that Merimac is only a partial translation of this Hobbit's name.
The similarity of 'Merimac' to 'Merrimac' or 'Merrimack' - place names in the United States - is a linguistic coincidence. Those come from Native American words meaning 'place of swift water', and would be quite inappropriate as names for the Brandybucks, which are generally Welsh in origin, or at least in style. (It's impossible to be sure, of course, that Tolkien wasn't influenced by those relatively familiar American names, but even if that is the case, the etymological origins cannot be the same.)
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