This is a toughie. First of all, the Denisars were in the US way before immigrants started coming through Ellis Island as you can see at
http://awt.ancestry.com/cgi-bin/igm.cgi?op=GET&db=jwible&id=I035603. Also, note that Frederick Denisar lived in Dublin, PA, which suggests an Irish connection, and that his son Daniel's family at
http://awt.ancestry.com/cgi-bin/igm.cgi?op=GET&db=jwible&id=I006951 was also recorded as Denison in the 1860 census. So an Irish connection is further suggested by the fact that Dennison is a common surname in Ireland. However, this is all circumstantial evidence and should definitely not be taken as the last word on the subject.