You didn't mention what religion they were but civil registration of Catholic marriages began in 1864 (from 1845 for other marriages). Therefore if they were married in a Catholic cerremony you'll have to rely on church records (if they exist). Otherwise, you can order the marriage certificate.
There are 4 children listed in IGI for Patrick Smyth & Eliza Ann/Elizabeth/Eliza Hollinger:
1. Mary born 1 Dec.1864 Castleshane, Co.Monaghan
2. Patrick born 8 Apr.1867 Co.Monaghan
3. James born 25 Jan.1869 Arclinda, Ireland
4. Michael born 3 Dec.1871 Rutherglen, Lanark., Scotland
Any children born before the start of civil registration of births in 1864 will not have birth certificates.
The LDS site also has a submitted record which says that Patrick was born c1833 at Cornagh, Tullycorbet and Eliza Ann c1840 at Cordlaugh and that they were married 14 Jan.1858 at Tullycorbet. This conflicts with your date of a marriage in 1852 which is also on LDS site.
If the Smyth family went to Scotland the birth certificates of any children born there should list the date and place of the parents' marriage. Also you should be able to trace the family in Scottish census records to see what ages are given for Patrick and Eliza.
