Given that Arthur seems to have been from Scotland and married in Ireland, it is likely, that is if Sarah had not been living in Scotland and they married there, that he married in his wife's parish. With the link for McKeown to Tamlaght townland, have you considered the Scullion families in the same townland as potential links to Sarah? Certainly, at the time of the Griffiths valuation in mid 1800s, as you may already have found, there were at least 3 male Scullions in Tamlaght and others in neighbouring townlands:
http://www.failteromhat.com/griffiths/antrim/rasharkin.htmAs you also may have already discovered, the 1851 census for Tamlaght townland is one of the few surviving for Ireland. There is no obvious sign of Arthur there nor, unfortunately, Sarah, albeit there are other Scullions. Always worth keeping it in mind though as may link in at some point.
On what records did you find Arthur's birthplace as Scotland? I ask, because I notice in Tamlaght at 1851 an Arthur McKeane and parents (mum Margaret), albeit born County Antrim, aged c25yrs, so born c 1826, so same year as your Arthur. I would not find it beyond the realms of possibility for McKeane to be a mistranscript/variation on McKeown, especially as there is no McKeane recorded in the Griffiths valuation for Tamlaght albeit there is an Arthur McKeown. Perhaps worth giving further consideration to this given that your Arthur named a daughter Margaret.
http://www.searchforancestors.com/locality/ireland/census1851/rasharkin.html