By some measurements the Viking presence in Ireland lasted from the early 790'ies to 1014. That is a span of aprox. 220 years, during this time a lot of things happened both in Ireland and Viking Scandinavia. The power balance in Ireland between what was primary Norwegian Vikings and the native Irish was not static.

In the 790'ies a 50 year period of raiding by the vikings took place in Ireland. During this period of 50 years the local Irish were seriously outclassed in military terms. The Norwegians primary advantage was their longboats, which the Irish had no answer to. The Vikings were able to raid up every significant river in Ireland were monasteries were sacked often multiple times. The Irish who had always been seen as on the outer edge of the world, were also lacking in terms of weapons compared to the Norwegian vikings. The Vikings were able to settle the mouths of all critical rivers, and thus control naval trade to and from Ireland. It is from the Vikings cities such as Wexford, Cork and Dublin emerges. The Norwegian where then able to somewhat organize their holding an establish a kingdom in Ireland.

The Norwegians never really extended their territory inland as the Irish knew how to use the terrain to their advantage, and Norwegian who would often win a pitched battle, if leading an expedition inland would often get ambushed. Instead their interests laid across the Irish sea in Viking England, which they saw a connection with. So the Norse kings of Dublin spend considerable efforts into uniting Viking England and Norwegian Ireland, but these efforts proved futile in the end. Instead the Norwegian focused their efforts at Dublin, they made Dublin the center of trade between Scotland, Wales and England.

The Norse in Ireland were at their height in the 9. century through to the middle of the 10. century. In the late 9. century the new arrivals from Scandinavia diminshed as the settlement of Iceland began, plus the Norwegian king Harald Finehair kept the Viking fleets at home, in order to unite Norway. The Norse in Dublin were briefly expelled from 902 by the Irish kings in Meath and Lenister until 917, when Sigtrygg II took back Dublin with a Norwegian fleet. Sigtrygg II reestablished the Norse kingdom in Ireland and conquered the hinterland of Dublin.

The power balance between the Vikings and the Irish shifted in the late 10. century. After the battle of Tara in 980 the Norse as the dominant power in Ireland ended. The battle of Clontarf in 1014 is generally seen as the end of the Viking age in Ireland, were the Norse were defeated again. The Norse remained in Ireland as rulers of the trading port cities they had either established or developed, but from then on they were usually subjects rather than the dominant power in Ireland.

So in order to answer your question, I would say the user you refer to might have a bit too simplistic take on the Viking presence in Ireland. The Viking age in Ireland lasted aprox. 2 centuries in which the balance of power between the Irish and the Norse shifted several times. To sum up, at first the Vikings had both naval and land supremacy, and were able to settle the crucial rivers. The Vikings interersts lay not in Ireland but with their percieved kinsmen across the Irish sea. As time progressed, the settlement of Iceland and the unification of Norway led to less reinforcements from Scandinavia. In the end as the Irish caught up in weapon technology they were able to decisively defeat the Vikings and subjegate the Norwegian rulers in Ireland.

Tl;dr: The Vikings held the upper hand at first and won battles at both land and sea, while they colinized several key points in Ireland. As time progressed and the attention in Norway shifted from Ireland to kingdom building the Vikings in Ireland were left to fend for themselves, and in the end the Irish overcame the Norwegian Vikings.

I haven’t seen the original post on TIL about Viking Ireland so I can’t directly address that [or charges of Irish nationalism of a poster by u/Zugwat] but there is the danger of oversimplifying the Viking experience in Ireland in the other direction. It would be a mistake to think that the Irish countryside in the Viking period was an underdeveloped oasis with vulnerable unprotected monasteries open to plunder with the Irish solely using ambush or surprise raiding tactics against Viking attacks. The major monasteries, which formed the backbone of Irish society, kept standing armies and fought pitched battles against each other long before the Vikings arrived on the scene. The Irish annals record more than 20 monastic pitched battles fought in the eight century prior to the Viking arrival. As Seán Duffy points out in Ireland in the Middle Ages, “It was not Irish society that adapted to cope with the Vikings so much as the reverse”.

Even in the early stages of the Vikings’ hit and run raids they came up against defeats. As early as 811 [the first Viking attacks were in 795] there was a massacre of Viking raiders by the Ulaid in the north. The following year they were slaughtered in the south by the Éoganacht. The Vikings did not have an easy time in Ireland and it would be a mistake to think so.

The coast of Ireland was not developed. The Irish tuatha [family group] lived in communities inland and it was it was easy for the Vikings to initially establish their strongholds along the south coast and on the east in Dublin. But they were unable to establish anything in the north or north west where the powerful Uí Neill dynasty – of the high-kingship - held power. The northern kings were traditionally more powerful and more influential than the other Irish kings. The Uí Neill dominated the region of Tara and everything north of that. Because the Vikings failed to dominate Irish life Dublin, and the other Viking towns along the south coast, came to resemble other Irish kingdoms and a form of co-operation developed between the Viking and some Irish kings.

But with the rise of the Dál Cais sept in Munster in the 10th century [a southern dynasty which would ultimately challenge for the High Kingship] the Vikings met with resistance and with raids on their stronghold in Limerick. The Dalcassians Brian Boru and his brother Mathgamain met and defeated Viking armies in inland pitched battles. In 967 the Annals of Inisfallen record a defeat of the 'foreigners' at Limerick by Mathgamain. The city was then burned by him. A year later when the Vikings attempted to go inland and take Cashel [the seat of the Munster kingship] they were roundly defeated by Brian and Mathgamain at the Battle of Sulcoit in modern day Kilkenny.