This answer has been discussed a couple times in the past, so I'll just provide a brief summation of some relevant points:
Counter factuals are often impossible to really give a complete answer to, because they inherently rest on something that did not happen. In this particular case, it is worth remembering that the Romans were not a Borg like species driven towards constant invasion and expansion--every addition to their empire had a particular reason to it.
Rome only came into sustained close contact with Ireland well after the era of its expansion was largely over. The period of imperial growth for Rome is largely between 202 BCE and 9 CE, starting with victory in the Punic Wars and ending with Augustus' clarification of the borders. These two hundred odd years saw Rome expand from a polity that encompassed Italy, parts of Spain and the Italian islands, to one that had absolute control over the entire Mediterranean basin and Western Europe to the Rhine. The only significant additions after that were Britain, which was conquered because Claudius badly needed a military victor to bolster his legitimacy, and Romania, which Trajan conquered because he was a warmonger (the addition of Mesopotamia lasted barely any time at all, and the border changes aside from that were largely formalizations of earlier de facto Roman control, eg Nabatea and Judea). In particular, the imperial court worked very hard to quell the miltarized status competition amongst the Roman elite class, which did not fully explain the expansion but certainly provided a driving impulse.
Ireland would have been very difficult to hold for very little gain. When we think of the Irish landscape, we tend to think of this, the rolling green hills which would have well suited Rome's legions and agriculture. But this landscape is intensely anthropogenic, created in recent times. In Roman times it would have been much more thickly forested, and covered in bogs and swamps of the sort that do not make for good invading. This can be seen in later history two, as part of the reason why the Norman occupation was, to an extent, a bit of a bust.
There just wasn't that much. Irish archaeology is still a bit of a developing field (not nearly as developed as archaeology in Britain) so it is hard to tell, but it seems as though the Irish Iron Age (say 500-500) was one in which social formations were considerably less complex than in early or later periods--in other words, a "dark age". Roman conquest tended to be highly succesful where there was already centralized states and hierarchical social relations which Rome would co-opt to its use. In areas with looser social formations, such as Libya, Wales and interior Turkey, Roman power was much more tenuous and difficult to maintain. Apart fro the simple logistical difficulty of conquering the island, it would have been extraordinarily difficult to maintain. And what is the point?
Peter Welles' Rome Beyond the Frontier has a chapter on Ireland in the period, which is well worth looking at.
