Thomas Welch (Welsh, Walsh, Walch) b. in Ireland marries Julia E. Shehan (Sheehan) before 1856

I am researching my maternal 2xGreat Grandparents, both born in Ireland.  It would appear they married before 1856 as the 1870 census shows them in Oil City, Venango Co., Pa. with their oldest child being Willie age 14 (born in Michigan).  Although, since there is 8 years between Willie and my Great Grandmother Katie It is possible this could be a second marriage for Thomas.  Does this ring a bell for anybody?