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I was not able to locate a marriage record for Martin and Margaret.
The surname Mulhaul was likely Mulhall. This site maintained by one of our volunteers https://www.swilson.info/sdist.php shows that Mulhall was found mainly in Leinster province in the southeast but also including Dublin. I've locate a number of Margaret Mulhall baptismal records but without her parents names, we can't confirm any records. By the way, the present Irish Ambassador to the USA is Daniel Mulhall https://en.wikipedia.org/wiki/Daniel_Mulhall You can also look up the location of Fitzsimons families in the 19th century.