In the 1800s houses in rural Ireland had no numbers. (In many areas that's still the arrangement to this day). The townland was/is sufficient to get a letter delivered and no other identifying information was/is needed. The numbering system in the censuses was simply the enumerator?s own private system and does not relate to any postal address nor tell you where the property was. The numbers vary between the censuses because a new enumerator had a different system. So, in this case, house 17 in 1901 is almost certainly house 6 in 1911. (The occupants hadn?t moved). The enumerators records have not survived and so there? s no way of ascertaining which house was where from census records.
To locate the house with any accuracy today, you need to look up the revaluation records in the Valuation Office in Dublin. That should tell you what plot number the Reilly house was. You can then go to Griffiths Valuation and locate that plot on their on-line maps. See:
http://www.askaboutireland.ie/griffith-valuation/index.xml?action=nameSearch
The property was near the modern L5604 minor road but, as I say, you need a plot number and Griffith?s valuation maps to find it with any accuracy.
In 1854 there were two John Reilly properties in Mellifont. (Plots 1 and 13a). Could be a relation. Without the valuation records it?s hard to say whether either is the property your family were in in 1901.
Unless someone on this site happens to known the townland very well, I doubt it would be possible to say whether the property is still there without going there to find out.