Carrickmacross is in the parish of Maghaire Rois (Magheross). Copies of the RC baptisms from 1858 – 1870 & 1878 – 1880, and marriages 1838 – 1844 & 1858 – 81 are in PRONI, Belfast. The records themselves are not on line. You either need to look them up in person or order a copy of the film in to your nearest LDS library.
I suspect that the above dates are as far back as the parish records go. That would not be unusual. Few RC parishes have records before the 1830s.