This is not the Alexander line that I am following but I have notes on a lot of Alexanders. In 1865 Andrew Alexander married Mary Patterson on 25th April at Carrickfergus, Co. Antrim. That may connect with your one (a cousin marrying a cousin). There are large numbers of Alexanders and Pattersons in Co. Antrim. It is very difficult make any connections with the amount of information you have provided.