Well it seems they do not call that offside under their own rules. to them it is a technical foul. See
www.leinstergaa.ie/_fileupload/Playing_Rules_of_Football_.pdf
"RULE 4 - TECHNICAL FOULS ... 4.9 For an attacking player to enter opponents’small rectangle before the ball enters it duringplay."
What happens in Aussie Rules if someone takes a mark and is preparing to have his kick and the defender standing on the mark waving his arms around and pulling funny faces at him oversteps the mark? Under the rules it is not offside but you could argue that it is offside because he went somewhere he should not have and incurred a penalty for doing so.
As far as I am aware, the term "offside" does not appear in the rule book for either codes, yet players can be penalized under the rules for being in the wrong place at the wrong time, but it is called something different.
Cheers
Doug