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I did a search on Roots Ireland for Co. Laois from 1845-1855 for a William McCann. Only one record came up, in the Doonane RC parish which is just to the west of Killeshin. The baptismal date (see transcription below) was in August 1850. This couple also had a James in 1848 and a Mary in 1852. The 1850 Griffiths Valuation head of household listing for Killeshin parish does show a Henry McCann in the town of Killeshin. So I would treat both records as possible leads.
You may also want to search www.irishgenealogy.ie which has Dublin city church records.
Let me know if you have questions.
Name:William McCannDate of Birth:11-Aug-1850
Date of Baptism:cordReNoAddress:NewtownParish/District:DoonaneGender:
Father:Edward McCannMother:Mary MurrayOccupation:
Sponsor 1 /
Informant 1:Thomas McCannSponsor 2 /
Informant 2:Murria Kelly