Researching a birth record

Hi, my name is Vicki Cathcart. I've been researching my husband's family tree.

I am trying to find my husband's Great Great Grandfather's birth record. His name was Matthew Cathcart.

Matthew married Eliza Thompson on June 1, 1882 in Coleraine. Denomination was Presbyterian, and his age at marriage was "full". 

His father's name on the marriage certificate was William Cathcart, but no information on his mother.

I had found a marriage of a William John Cathcart marrying  Eliza Wilson. It makes sense as William's birth date was 1824 and Eliza's birth date was 1833. They were married on January 18, 1851.

William John and Eliza Wilson were married in Kirkinriola, Antrim, Ireland.

In the 1901 census Matthew was 45 and was born in Co. Antrim, which would make him born about 1856.

In the 1911 census Matthew was 56 and again it only says born in Co. Antrim which would make him born in 1855.

So, I would assume, that his year of birth was 1855/1856.

I am just trying to confirm his birth date and his mother's name. I would love to find Matthew siblings as well as I am writing to a distant cousin, but we both can't seem to figure out where we both join on the family tree. My thought was that her line was one of Matthew's brothers or sisters, but I can't find them right now.

Thank you for your time.

Vicki Cathcart