It can be quite the puzzle with people with similar or the same names, trying to figure out if they're yours or someone else's. :)
There are court and prison records digitised and online that may be of help to you, especially since convicts that were to be sent to the penal colonies would reside in a prison for some time until their ship came, so both of these records may be of help to you.
We have a fairly extensive list of resources
that you might like to look into as well, they may give you some extra ideas of where to search.
The Irish prison registers
collection, now online, covers the full range of detention facilities available from 1790 to 1924. Of course, they're not complete, but they give you a good opening there.
A quick search shows 63 men by the name of John Devane incarcerated in Tipperary, with many without a known date of birth. There is no John Devane listed as being born in 1818. Of course, this may mean very little, as he may have been incarcerated in a prison nearer a port or elsewhere to await his deportation.
Another good place to look are the Petty Sessions Court Registers
. The Petty Court was the lowest court of the land, so if he was convicted there, he may be found in these.
Your John having fibbed a little about his date of birth would not be rare if age was a factor in being allowed to immigrate and find work.
Hopefully some of our experts will chime in and be able to give you some good advice later as well.
Wishing you a wonderful day,