When thinking of conquest one must always see the use of the conquered land and the effort it takes to take over said land and to control it. We can't be sure why exactly the Romans never sought to conquer Ireland but we have some ideas.

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Tacitus writes about Ireland in his book about his father in law, Gnaeus Julius Agricola, as he was a general in the Roman army and Governor of Britain. In said book De vita et moribus Julii Agricolae - On the life and character of Julius Agricola) he lays out the conquest of british soil and the pacification of said land.

We know of Agricola that his time as governor in Britannica was very succesful, as he managed to bring peace to large parts of Wales and England. Notable in this context is the caputure of the island of Anglesey, just off the coast of Wales, which was known for its resistance against the romans; due to the lack of ships at this time of his campaign, he made the finest men of his auxiliary troops swim to the islands, where the starstruck inhabitants who had been looking for ships offered peace (Tacitus: De vita et moribus Julii Agricolae - chapter 18). His time there also included very succesfull operations against the caledonians (notable here is the battle of Mons Graupius, where Agricola won against the Caledonians who were at least 15 000 men strong only using his auxiliary troops and with minimal losses), furthermore he strengthened control of the Britannica by building new fortressses and expanding the street system.

However, we have to be cautious of these claims, as they are made by a Roman, in whose interest it is to make the Roman empire - and furthermore: his father in law - shine in an even brighter light. So reports such as the one of the battle of Mons Graupius have to be taken with a pinch of salt, as overexaggeration of Roman success was in Tacitus best interest.

It's undoubtebly true however, that the time of Agricola as governor of Britain brought a new level of stability to the island, afterwards he was renowned for his success in this province, also explaining his long tenure as governor (AD 78 - 85).

Now, let's get to the interesting part.

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Chapter 23: "The fourth summer he employed in securing what he had overrun. Had the valour of our armies and the renown of the Roman name permitted it, a limit to our conquests might have been found in Britain itself. Clota and Bodotria, estuaries which the tides of two opposite seas carry far back into the country, are separated by but a narrow strip of land. This Agricola then began to defend with a line of forts, and, as all the country to the south was now occupied, the enemy were pushed into what might be called another island."

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Chapter 24: In the fifth year of the war, Agricola, himself in the leading ship, crossed the Clota, and subdued in a series of victories tribes hitherto unknown. In that part of Britain which looks toward Ireland, he posted some troops, hoping for fresh conquests rather than fearing attack, inasmuch as Ireland, being between Britain and Spain and conveniently situated for the seas round Gaul, might have been the means of connecting with great mutual benefit the most powerful parts of the empire. [...] I have often heard him say that a single legion with a few auxiliaries could conquer and occupy Ireland, and that it would have a salutary effect on Britain for the Roman arms to be seen everywhere, and for freedom, so to speak, to be banished from its sight.

To anwer one of your questions: The Romans definitely had knowledge of Ireland. Agricola even landed there and waged war. So at least some Roman figures saw Ireland as a useful addition to the empire, especially as another factor to bring stability to the whole of Britain (freedom banished from its sight) and as a way of connecting Britain with Spain around Gaul. Furthermore he states that a single legion with auxiliaries would have been able to take control of the island. Why exactly is it that the Romans never attempted to control Ireland? Parts of the answer can be found in the rest of chapter 24:

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Its extent is small when compared with Britain, but exceeds the islands of our seas (Mediterranean sea) . In soil and climate, in the disposition, temper, and habits of its population, it differs but little from Britain. We know most of its harbours and approaches, and that through the intercourse of commerce. One of the petty kings of the nation, driven out by internal faction, had been received by Agricola, who detained him under the semblance of friendship till he could make use of him.

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First of all: Britain and Ireland were not seen as the most desirable place to Romans. The climate was rough, Ireland was full of woods and there seemed little to be gained from it regarding materials or rare trade goods. So similar to how the Romans opted to not try to control Germania after the defeat at Teuteburg Forest and the campaigns by Germanicus to avenge said battle and retrieve the eagles convinced the Romans that it was not worth the resources they would have needed to pour into a conquest.

Tacitus goes on to describe the temper as similar to the one of the britons, who had attempted uprisings more than just once and almost made Nero withdraw his troops from the province (see below Suetonius: Nero chapter 18).

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The successes of Boadicea, queen of the Iceni, who inhabited Derbyshire, were probably the cause of Nero's wishing to withdraw the legions; she having reduced London, Colchester, and Verulam, and put to death seventy thousand of the Romans and their British allies.

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Thus we can guess that one reason why the Romans never took control was the fear of another "problem zone" in the empire.

In the following years, other areas were seen as more important to stabilize, as after the revolt of Boudica and the succesfull governance of Agricola the province was mostly peaceful. Other parts of the empire however were not and the attention lied on revolts in Germanica magna (AD 89 by Antionius Saturnius), Dacia (AD 85 - unsuccesful military campaign by the emperor Domitian, later conquest by Trajan) and the war in Parthia by Trajan in 113.

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After the expansion hungry Trajan had died, Hadrian followed and instead of continuing Trajans efforts in enlarging the empire rather tried to stabilize both the economy and the provinces. In Britain this happened by building the hadrian's wall, shaping the borders of the province for years to come.