I dont know if you've already checked, but records for the RC parish of Crossabeg are available in the National Library in Dublin on microfilm Pos. 4251. The parish is listed on the NLI index as Crossabeg and Ballymurrin, and the corresponding film only seems to include baptisms, and from 1856 to about 1880
There's a map of the RC parishes in Co. Wexford on the Irish Times website which might help locate other nearby parishes :
Wexford, RC Parishes The parish links on the website show the sources, but in general most of the surviving RC records are available on microfilm in the NLI.
Given the dates involved it should be possible to find the two marriages you mentioned (James Walsh/Margaret Pender & John Keegan and Mary Sinnott) on the civil index to locate certs for these to track further details.
see also :
2 Minute Introduction to Irish RecordsShane