Hi Ann

Have a look at scotlandspeople website. I had a quick look and could find no Daniel LAW births / baptisms that would fit your Daniel but there was a Donald born / baptised 2 July 1796 to Peter LAW and Jean McELLAR, Dunoon and Kilmun, Argyllshire. I seem to recall that Donald is the gaelic equivalent of Daniel. I could find no record of Daniel / Donald marrying Martha Muir in Scotland. There was however a Daniel LAW aged 35 in Rhu (or Row), Dunbartonshire. I didn't check the details so I don't know if this is your man.

Regards,
William ( Will ) Law