Glasnevin Cemetery was opened in 1832 and they now have records available online. At present only data after 1898 is included but they plan to add the older data - which might include details for Maria.
http://www.glasnevintrust.ie/genealogy/Records for the RC parish of St. James are available on microfilm in the National Library and in the Pearse St. Library in Dublin. Some of the records for this parish are included in the online database on the Dublin City Library website at :
http://dublinheritage.ie/index.php I dont see a listing for the marriage of Peter & Maria. There is an earlier Casserly marriage listed in a different parish :
Thomas Casserly Maria Dillon
date : 21-04-1754
parish : St Andrews
plus two Baptism's in st. Mary's Parish that look like they could match :
21-12-1837 Casserly James Bernard
29-09-1839 Casserly Susannah
do any the details you have (e.g. census returns, death certs etc.) show a detailed place of birth for Peter ?
some details from various directories :
Pettigrew & Oulton - 1845
Casserley Peter, attorney, 16 Stafford st
Slater's 1846
Casserley James, barrister, 1 Nelson st
Casserley Peter, attorney, 16 Stafford st
Thom's 1850
Peter Casserley, attorney, 16 Stafford st - res. Dollymount
also listed under Clontarf/Dollymount as Peter Casserly esq
Thom's 1852
J. Casserly, barrister, 93 Upr Dorset Street
Miss. Casserly, 36 Lwr Mountpleasant Ave., Rathmines
Peter Casserly, solicitor, 16 Stafford St & Dollymount
Barrister section - J. Casserly, Dorset St. - called to the bar 1839
Shane