There are 170 Costello's still in the telephone book for Tipperary. None at a place named Glengar. I tried to find the closest big town.
Glengar was a Townland in the Parish of Doon, in the Union of Tipperary which means within a 12 mile radius of the Town of Tipperary.
For births after 1864 you should find them onwww.familysearch.org
Prior to 1864 would only be a Parish records which are available via an online not for profit Heritage Society that does charge a fee to search.
I didn't find them in the 1901 census but did see the 1911
and see Patrick and Ann had been married 27 years, had 7 children and 6 were living. This puts the marriage about 1864 and it could be listed on familysearch, however, I was unable to find it, so possibily it was just prior to 1864.
I did find a Thomas Costello born 1864 Carrick on Suir,
no details other than that.
Tipperary is a member of a Heritage Foundation with birth and marriage records both before and after 1864. This is a not for profit organization but a fee is charged. You can
get certified copy of a civil registration from
the General Registry, Dublin www.groireland.ie
there are 170 Costello's still in the phone book for Tipperary but no name Glengar that I can see, I only looked at page one, but addresses given are sufficient for mail delivery. there are 17 Sheehy's.