Brenda:
There are no 1821 census fragments available for Co. Leitrim. Not sure what source would have shown Isabella living with George and his family.
The 1834 Tithe Applotment listing for Carrigallen ciivl parish did show a George Thompson in Lugnagon townland (shown as Legnagun in the Tithes).
http://titheapplotmentbooks.nationalarchives.ie/reels/tab//004587412/004...
You did not mention the religious denomination but if the family was Church of Ireland then no records would be available because the records were destroyed in the 1922 Public Records Office fire in Dublin.
Roger McDonnell