Hibernia

[Tiberius Clodius Corvinus]

I guess it was kind of cost-benefit calculation. Ireland wasn't really worth to be conquered and may be considered not big a enough a threat to be engaged. To my knowledge even Britannia never paid the cost of the occupation and was basically conquered for prestigious reasons. The few valuable goods and ressources of Ireland were probably easier to get by means of simple trade.
Additionally Ireland would've been even more difficult to supply with troops.

There is a thread on RAT that deals with the topic, but I don't find it right now. Perhaps someone else is more lucky in digging it up ...