Mac Casarlaigh
Mac CASARLAIGH—IV—M'Cassarlie, MacCasserly, Casserly, (Cassidy); 'son of Casarlach' (an Irish personal name, in use as late as the beginning of the 17th century); an old Roscommon surname. Cumumhan Mac Cassarlaigh was among those who fell at the battle of Athenry in 1249.
Alphabetical Index to Irish Surnames