To my knowledge, there is no translation of this tale appart from The Siege of Knocklong/Forbhais Droma Damhghaire, Irish Amer Book Co-1993 by Sean O Duinn. The following is a summary by O'Curry in Lectures on the Manuscript Materials of Ancient Irish History.
f the Forbasa listed in the Book of Leinster there is one more so remarkable, that I would make room for some account of it, if it were possible — namely, the Forbais Droma Damhghairé, by king Cormac Mac Airt, against Fiacha Muilleathan, king of Munster, about the year of our Lord 220. Drom Damhghaire was the name of a ridge or hill in the county of Limerick, since Cormac's time (and still) called Cnoc Luingé, or Knocklong, from the tents set up there by Cormac, who encamped upon the spot. The following is shortly the history of this Forbais : —
Cormac's munificence was so boundless that, at one time, his steward complained to him, that, although there were many claimants and objects of the royal beneficence, there was nothing for them, as all the revenues appropriated to such purposes were exhausted. Cormac, in this extremity, asked the steward's advice as to the best means of replenishing his stores. The steward, without hesitation, said that the only chance of so doing was in demanding from Munster the cattle revenue of a second province ; that it contained two distinct provinces, but that it had always escaped paying tribute but for one, and that he ought to call on them for the tribute of the other.
Cormac appeared to be well pleased with this suggestion, and immediately despatched couriers to Fiacha Muilleathain, the king of Munster, demanding tribute for the second division of that province. The king of Munster received the monarch's message in a fair spirit, and sent the courier back with an offer of ample relief of Cormac's present difficulties, but denying his right of demand, and refusing to send a single beef in acknowledgment of it. Cormac having received this stubborn message, mustered a large army and all his most learned Druids, marched into the heart of Munster, and encamped on the hill then called Drom Damhghairé, or the " Hill of the Oxen".
Having established his encampment, he consulted his Druids on the best and most expeditious means of bringing the men of Munster to terms. The Druids, after debate among themselves, assured the monarch that the surest and most expeditious mode of reducing his enemies would be to deprive them and their cattle of water, and that this they were prepared to do on receiving his permission. Cormac immediately assented, and forthwith the Druids by their spells and incantations dried up, or concealed, all the rivers, lakes, and springs of the district, so that both men and cattle were dying of thirst all round them.
The king of Munster in this extremity took counsel with his people, and the decision they came to was, not to submit to Cormac, but to send to the island of Dairbré [now called Oiléan Darairé, or Valencia], on the western coast of Kerry, to Mogh Ruith, the most famous Druid of the time (who is said to have studied Druidism in the East, in the great school of Simon Magus), to request that he would come and relieve them from the terrible distress, which they well knew had been brought on them by Druidic agency.
The ancient Druid consented to come and relieve them, on condition that he should receive a territory of his own selection in that part of the province, with security for its descent in his family for ever. His demands were granted, and he selected the present barony of Fermoy in the county of Cork (where some of his descendants survive to this day, under the names of O'Duggan, O'Cronin, etc.). The Druid then shot an arrow into the air, telling the men of Munster that water in abundance would spring up wherever the arrow should fall. This promise was verified ; a rushing torrent of water burst up where the arrow fell ; and the men of Munster and their flocks were relieved.
The Munster men then fell upon Cormac and his hosts, routed them from Cnoc Luingé, and followed them into Leinster, scattering and killing them as they went.
Sources : Eugene O'Curry, Lectures on the Manuscript Materials of Ancient Irish History