That would be playing fast and loose with the original sentence. You're meant to give an accurate translation, and using 'online' wouldn't give you that.
Online = ar líne. You can be online without being on the Internet... Granted, you cannot be on the Internet without being online... but then you could say that you saw it on hearth...
Why isn't "t-" before "idirlíon"? Is it because of the preposition?
It is called the "dative case", used after most prepositions. In rare cases, the form of the word changes too: in Éirinn...
(Corrected to "dative")
Is " ar an idirlíon" not the dative case? The accusative case in Irish is not all that relevant ,I was told, as it follows the same rules as the nominative.
It is, your are right.
The dative case is not much more relevant, with some few exceptions. Most of the changes in the dative are some initial consonants.
In this case... the three are identical except for that initial "t-" in the nominative.