He was King of Thomond from 1360, when he deposed his uncle, King Diarmaid, until his death in 1369.

From book "Kings and Queens of Britain" by David Williamson, p234: King of Thomond 1343-1369

The History and Topography of the County of Clare by James Frost
Part I. Topography of Thomond Chapter 1. General Division
Rent-roll of O’Brien, King of Thomond, in the fourteenth century
The original, in Irish, will be found amongst the ancient Irish deeds published by Hardiman in the 15th volume of the Transactions of the Royal Irish Academy. Although it bears no date, it may, with some certainty, be assigned to the middle of the 14th century.
“O’Brien’s Rental.”
“This is the amount of the rent of O’Brien from Kilrush to Clondagad (exclusive of Lisconnellan, out of which he has two bushels of wheat yearly), viz.: 3? marks, and 40 marks over and above his footmen and sportsmen. These are the lands out of which said rent arises, viz.: 1 oz. of gold in Tullyderry; 1 oz. gold in Carrowmore; 1 oz. of gold in Kilrush; 1 mark in Ballyerra; 20 shillings in Kilkevin; 2 ounces and 3 marks in Killofin; one oz. of gold in Ballykett; 2 marks in Moyadda; Kilcarroll; Burrane; Moylougha; Crossmore (a Cillfíne); 20 shillings in Tullycreen; 20 shillings in Derrycrossane; an ounce of gold in Binvoran; an oz. of gold in Lack West; 20 shillings in Drumdigus; an oz. of gold in Cloonarass; an oz. of gold in Ballygeery; 20 shillings in the two Ballyannas; an oz. of gold in Methany (Meitanaigh); an oz. of gold in Clonkianly; an oz. of gold in Erribul; an oz. of gold in the Effernan; 20 shillings in the two Shanakeas; an oz. of gold in Caheracon; 20 shillings in Coolmeen; 20 shillings in Cloonulla; an oz. of tribute and an oz. of gold in Craghera and in Cloonsnaghta; ten pence tribute and oz. of gold in Cloonkett; ten pence tribute and an oz. of gold in Bolooghra; ten pence tribute and an oz. of gold in Cloonboirne; an oz. of gold in Glenconaun; ten pence tribute and twenty shillings in Ballyvohane of the clan Kennedy; 20 shillings rent and an oz. of tribute in Ballyvohane from the descendants of Teige MacMahon; an oz. of gold in Cappanavarnoge; one shilling and nine oz. in the one-fifth of Inishmacowney; one shilling and nine oz. in Leckannashinnagh; eighteen pence tribute in Lisconnellan; two shillings tribute in Ballynagard (now Paradise); two shillings in Crovraghan; Ballylean; 20 shillings in Inishmore; a mark in Roscliff and Ballinacally; 20 shillings in Liskelly O’Keilly; 20 shillings in Liskelly O’Nuins Liskologe;[2] three oz. in Crappoge;[3] 18 oz. in Inishtire; and an oz. of gold in Lack mac Brien Gankach.[4]
“This is the amount of the rental of O’Brien from the lands lying westward of Traig-na-Croise, in West Corcabaskin, viz.—nine marks of rent, and 20 shillings tribute over and above his footmen and sportsmen, i.e., two marks in the two Moveens; a mark in Faelan; a mark in Carrowbane; a mark in Trusklieve; a mark in Bellia and in Garraunatooha; eight pence and an oz. of gold in Cloghaunsavaun; eight pence and an oz. of gold in Oughterard; twenty shillings in Fodry. These are the tributary lands of O’Brien in that country: six pence on Kilbaha; 4 groats on Kilcloher; one mark and one oz. on Rahona; 20 shillings on Rinemacaderig Kilcredane; 2 oz. on the Killybegs; 4 groats on Rehy west; 14 pence on the two Treans and the Cally;[5] 2 marks on Moyarta; ten pence on Doonaha; 14 pence on Rehy east; 14 pence on the Cross; 6 pence and 3 oz. on Caheraghacullen; one shilling on Clooncarran; one shilling on Kilfearagh; 14 pence on Ballyonan; 14 pence on Kilkee (Cillchaidh); 2 shillings on Lisluinaghan and Lisdeen; a groat on Kilkurn; an oz. on Aillinclogher; six pence on Roinmicnisg; and one shilling on Lisheencrony.
“This is the amount of the rent of O’Brien out of Corcomroe, over and above the gallowglasses, royalties, and sportsmen, viz.:—twenty-six marks and eleven pence, and that sum is exclusive of the rent of Tuaith Glae (Tuaith Glae).[6] The land out of which said rent arises are Inchovea, Bally Mac Donaldbane, Carrowgar, Ballyrohan, Ballykinvarga, Ballyshanny, Bloady, Ballygoonaun, Craevdergan, Carrowduff, Caheraderry, Rannagh, Ballyfaudeen, Clooney, Ballyvorda, Shedan, Carrowgar, Longfort, Glenmacconnor-na-cally, Cullinagh, Derrymore, Moananagh, Teerlaheen; Cahersherkin, Ballyculleeny, so that there were forty-three quarters, seven of which were subject to a mark each.”
“This is the amount of the rent of O’Brien in Tuaith Glae, over and above the gallowglasses, royalties, and sportsmen, viz.:—ten pence and 5 oz. and 12 marks; and the immunities of the Clancy family [7] are charged on same. The lands subject to that rent are Ballynahown, Carrownycleary, Creggycorridan, Carrowgar, Glasha, Ballycullaun, Doonmacfelim, Ballyvoe, Ballynalacken. The immunities of the race of Clancy, with ? a mark of the rent of O’Brien, are in Creggycurridan, so that there are five pence and five oz. on each of these 14 quarters.
“This is the amount of O’Brien’s lordship under the stewardship of Maccon Cairgi,[8] arising from the moiety of Burren, over and above beeves and swine, [9] viz.:—23? marks, (and there are ten marks immunities included in them), fifty-one half marks belonging to the wife of O’Loghlin, and a twenty in Finnaveara. The number of beeves and swine under the stewardship of the said stewards is six beeves and six swine, and gallowglasses, royalties, and privileges are not reckoned. These are the ploughlands subject to these dues, viz.:—Bally-g-martin, Cahermaan, Ballyganner, Caherpolla, i.e., Lisinorahaun, Lisheeneagh, Rannagh, Cappagh, Knockan, Urling, Shessia O’Donnell, Creevagh, the ploughland of Brian’s mother, Sheshymore, Fanygalvan, Cahirmackirrilla, Tulglaishe, Meggagh, Eantybeg, Ballyustad.”
“The amount of the lordship of O’Brien under the stewardship of the family of Camluas [10] from the other moiety of Burren. The number of beeves and swine under the management of these is six beeves and 20 shillings of swine-money, over and above gallowglasses, royalties, or privileges of O’Brien. These are the lands subject to said charges, viz.: Ballymahony, Ballymurphy, Caltraghs, Glenslaid, Ballytoole, Formoyle, Caher, Lismahlia, Murroogh, Fanadfodhman, Derreens, Lisflattery, Ballymaelcher, Lisgoogan, Ballymultan, Ballycahill, Dangan, Knockanteemoragh, Kilbreac, Lisnalougherny, Rooda, Corranroo, Ballygastell, Feenagh, Dangan, Ballyvaughan.
“These are the proportions of the Camluas family in their claims of stewardship over the lands, viz.:—the townlands of Ballyconry, Lisbercan, Caherlappan, and Cahermakerrila.”
The foregoing rent roll refers to a part of Thomond only, but it is highly probable that O’Brien, in his character of king, had similar claims on the several other districts which constituted the kingdom of North Munster.