Had a patent to hold a fair, dated 1607, at Gortinsiguara, and was made a freeman of Galway in 1611. On the breaking out of the insurrection in 1641, he appears to have aided the Earl of Clanricarde, and by his order hanged two cow-herds for having killed a minister in the country.54

From "A New History of Ireland," edited by Moody, Martin & Byrne, vol 3, ISBN 0-19-821739-0, 1976, p 253-5:
[About 1633] A planning committee had been set up to prepare for the plantation of Connacht before parliament ended. The report that it presented in May embodied a revolutionary change of policy: in recommending that claim should be made to an estimated 4,000 quarters, of which 1/4 should be confiscated and the rest surcharged, no distinction was made in favour of the Old English county of Galway. The acceptance of this proposal extended the principle of plantation from colonisation to recolonisation and placed at hazard the privileged status upon which the entire Old English community depended for the protection of its property. The decision made it certain that the scheme would meet unprecedented opposition, for those who were now to be treated as if they were Irish, as the late earl of Ormond had expressed it, did not suffer from the political disabilities of the Irish: they were acknowledged members of the political nation, and had access to its means of protest.
Among them, moreover, was Richard Burke, earl of Clanricard, lord lieutenant of the town and county of Galway, who lived in England and moved familiarly at court. His influence there was attested not merely be his being the only Catholic to hold important office in Ireland but by an English viscountancy (of St Albans) and an official immunity from recusancy proceedings. It was his brother, Lord Clanmorris, and his steward, John Donnellan, who took the lead in organising the Galway landholders in defence of their estates as soon as the government's intentions were revealed by the initiation of proceedings in the court of chancery towards the end of June.
On their behalf, a search for records of title was being undertaken in England while Wentworth was progressing ceremonially across Ireland to preside over a series of inquisitions, at Boyle on 9 Jul, Sligo on 13 Jul, and Ballinrobe on 31 July, at which the king's title was found to the counties of Roscommon, Mayo and Sligo. At none of these sessions was there any pretense at a balanced investigation of the state of titles. The official view, bullyingly presented by the lord deputy to carefully selected juries, was that the crown title was incontestable and the inquisitions unnecessary, their sole purpose being to allow local occupiers to establish a claim upon the king's gratitude by admitting his rights with a good grace; the implication, that if his title were not found all the land would be resumed by other means, was clear.
In Galway, were inquisition proceedings began in Clanricard's Portumna residence on 14 Aug, these tactics were unsuccessful. Outraged defiance was buttressed by the discovery of additional proofs of title in England and the jury found against the king. But the ingenuous notion that plantation could be prevented by legal process was quickly dispelled. The jurors were at once bound over to appear in the court of castle chamber. The sheriff who had chosen them was heavily fined and imprisoned, investigations were instituted to determine whether a charge of conspiracy was warranted, and resumption proceedings were begun in the court of exchequer.
The Galway landholders, however, remained unitedly unrepentant. Ignoring a proclamation which called for individual voluntary acknowledgements of the king's title, they delegated 3 agents, Patrick Darcy, Richard Martin, and a prominent native Irish landowner, Sir Roger O'Shaughnessy, to present their case to the king. The Galway approach was largely an appeal to royal honour, shrewdly supplemented by an offer to double crown rents in the county. Their prospects of success, however, rested less upon the merits of their petition than upon the influence of Clanricard. If successful, they would not only frustrate the plantation of Galway, in which a punitive confiscation of half the land had been decided upon, but would also destroy the new-found authority of the Dublin government. Wentworth therefore insisted that the 3 agents should be received as conspirators rather than as petitioners and sent back to Dublin under arrest. If there were any doubt that the lord deputy would prevail, not the 3 agents, it was removed by the death of Clanricard early in November, shortly after the arrival of the agents.
They were given a formal hearing, quickly dismissed, and instructed to report to Wentworth in Dublin, but they were slow to leave. That they were afraid of Wentworth's wrath is clear: Martin went into hiding in London, O'Shaughnessy 'lingered' on the road to Bristol, and Darcy requested permission to live in England. The lord deputy was seriously embarassed: the delay in the return of the Galway agents had obscured the failure of their mission. At the end of May 1636 the Galway jurors were convicted of both conspiracy and wilful refusal to find the king's title, fined £4,000 each, and imprisoned. They remained defiant.

Abstract of Deposition in a cause in the Chancer of Ireland, wherein Fulk Comerford was Plaintiff, and Roger O'Shaghnes, of Gort-Inchigorye, in Galway county, Defendant, touching the town and lands of Cappafennell or Capperell, in that county, A.D. 1615:
Donnell O'Halloran of Gilloconry, in Galway county, husbandman, deposed that Sir Roger O'Shagnes was son and heir of Sir Dermot--that Sir Roger was54 married to Honora ny Brien, by whom he had four sons:--1st John, born about four or five years before the marriage, as were also two daughters, Joan and Margaret; and 2nd, William; 3rd, Fergananym; and 4th Dermott, born in marriage; that William was married, but died without male issue, and Fergananym died unmarried; that John O'Shagnes conveyed all the lands in O'Shagnes county to Sir Geffrie Finton for the sole consideration of Sir Geffrie maintaining the title of John against Dermott; that John was continually disturbed in his possession by Dermott, the defendant's father; that Dermott, after the death of his two brothers, and in the lifetime of John, enjoyed the greatest part of the lands of which Sir Roger had died seised, and that John was always reputed to be a bastard; that Sir Roger, the defendant's grandfather, enjoyed these lands (viz., Cappafennell), and had tillage there, having had at one time fourteen score of reapers in harvest cutting, of whom deponent was one. Depositions to the same effect were made by the following persons, viz.:
--Knougher Crone O'Hyne of Ledygane, gent, 100 years old and upwards.
--Richard Burke, of Rahaly, in Galway county, 64 years, or thereabouts; who added that he had seen an Order of Council made by Sir Henry Sydney, between Dermot and William, brother and son of Sir Roger, ordering that William should enjoy O'Shagnes' lands to him and his heirs male, remainder to Dermot, Sir Roger's brother.
--Margaret, Countess Dowager of Clanrickard, 80 years old, and upwards, sister of Honora, wife of Sir Roger, who added that they were married by a dispensation from Rome.
--Manus Ward, Dean of KilmacRoweth (Kilmacduach), 80 years old or therabouts; who added that he knew of the controversy between Dermot and William O'Shaghnes as above mentioned, wherein Dermot endeavoured to prove Sir Roger's sons bastards, because their mother was abbatissa, and could not be wife.
--Sir Tirrelagh O'Brien, of Dowgh, in Clare county, Knight, nephew of Honora ny Brien.
--Donell O'Heyne, of Killaveragh (Kinvara), freeholder, aged 80 years.
--Richard Lord Bermingham, Baron of Athenrye, nephew of sir Roger by his mother.
--Tirlagh Roe MacMahowne, of Clare county, Esq., 44 years old; who added that he knew the defendant's father, Dermott, to have been in suit with John O'Shagnes, and to have held Gort-Inshygory, the Newton and Ardemoylenan, during Sir John's lifetime, as heir of the body of Sir Roger.
--Nehemias Folan, of Balladowgan, in Galway county, Esq., 60 years old; who added that Dermott Reagh O'Shagnes, brother to Sir Roger, being servant to the Earl of Leyster, having come from England after Sir Roger's death, brought in question the legitimacy of Sir Roger's sons by the Lady Honora, at which time, during Sir Henry Sydney's Government, it appeared that the said Honora was a professed nun when the said Sir Roger had the said John by her, and that afterwards a dispensation was procured from Rome for their marriage.54

Sir Roger possessed a castle near Timolegue Abbey, in Cork, where his family then were residing and besieged, which is shown by the following extract from a letter written by the Marquis of Clanricarde to Lord Inchiquin:--"The bearer, my noble kinsman Sir Roger O'Shaughnessy, has by my licence taken his departure out of the Government into Munster, to take care of his lady, family, and estates, in these parts, which, by reason of his long absence, doth, and may suffer by the general unhappy distemper in this kingdom. I could not let so much worth and merit pass from me, without giving your Lordship notice, that in his own person, his son and followers, he hath constantly, and with much forward affection, being present and assisting to me in all my proceedings and endeavours for his Majesties service." The son alluded to was Dermot, who had raised 50 foot soldiers for Clanricarde; and Sir Roger's brother was a captain in this levy.54

Sir Roger wrote a letter from his Castle of Fidane in 1647, to his daughter Julia, wife of the Chief of Clancahill, in Cork, of which the following proves that Sir William Burke, or De Burgh, Knight, had issue by Joan, daughter of Sir Dermot O'Shaughnessy, Knight:-- "For my verie loveinge Daughter Mrs. Gyles Donouane, at Cstledonouane. Daughter, I have received yours of the eighteenth of Februarie last and as for your troubles you must be patient as well as others and for my parte I taste enough of that fruit; God mend it amongst all, and send us a more happie tyme.