You could say both, but operations were mostly locally devised and carried out without much influence from a central chain of command. There was certainly well co-ordinated national action, such as on the night of 3/4 April 1920:
Assaults on local police stations with rifle and bomb were continually beaten off (nearly two-thirds of raids against occupied barracks were unsuccessful in 1920). However, many garrisons were withdrawn from small isolated outposts, and on the night of 3-4 April 1920 over 150 of such posts were simultaneously burned all over Ireland together with income tax offices in sixteen different counties. By the end of the year 510 unoccupied ‘barracks’ had been destroyed. (Kee, p102) (Many of the 600 or so RIC ‘barracks’ in Ireland were simply ordinary two-storey houses in village streets)
Later in the war there were 'Flying Columns', particularly out in areas like Cork, with IRA units operating deep in the countryside, carrying out ambushes on police and military units, raiding RIC barracks and Unionist farms, with some degree of oversight and support from 'Brigade'. You need to be careful about reading too much into the declared military structure of the IRA, however. Sheehan (p54) mentions Captain Patrick Collins, G Company, 2nd Battalion, 1st Cork Brigade IRA which gives the impression of a very large force, but the numbers involved were nowhere near those you would find in a standard military company or brigade. Although while the numbers of actual fighters were small, they did have significant support:
“The Government is not up against a small band of extremists, but is faced with a considerable proportion of the manhood of Ireland.” Letter from General Sir Neville Macready to Sir Henry Wilson, Chief of the Imperial General Staff, 19th February 1921 (Sheehan, p63)
Note how this private sentiment from a senior officer contrasts with official propaganda coming out of Dublin Castle:
The majority of the Irish people are as disgusted with the doings of the IRA as everybody else but they are in the unfortunate predicament of being too frightened to say so. The extremists who are at the bottom of the whole wretched business make murder a hobby and for an Irishman or an Irish woman to openly disagree with them is simply a novel way of committing suicide.” Press statement, 1920, Propaganda Department, Dublin Castle (Sheehan, p61)
Although it is significant that throughout the 1919-21 War of Independence the entire Catholic church hierarchy (Cardinal Logue, the three archbishops and the vast majority of parish priests) condemned bloodshed by the Volunteers as crimes and offences against the law of God (Kee):
“Who has authorized a small band of unknown, ignorant persons to meet in secret and decide that the life of a fellow human being may be taken lawfully… The Irish people will not approve of bloodshed, and the freedom of martyred Ireland will not be achieved by midnight assassination.” Father Gleeson, parish priest of Lorrha, on the shooting of RIC Sergeant Brady while on patrol duty in Lorrha, Tipperary, Irish Times, 9 September 1919
“We are not at war. If we are, it should be declared or waged by the public and competent authority and not by a few individuals.” Dr. Joseph Hoare, Bishop of Ardagh and Clonmacnoise, at Mass in Longford, Irish Times, 7 September 1920
The support of the populace was also not necessarily achieved without threat:
“Persons must remember that an attitude of neutrality is inconsistent with loyalty and will render them punishable.” Notice in the Irish Independent, 3rd January 1921 (Sheehan, p103)
To get back to the question, though, most rebel action does seem to have been fairly independent, carried out by very small groups with little overall supervision, supply or co-ordination. These small local actions would range from intimidation (such as digging a 'warning grave' on the land of a suspected informant), to firing a few shots at police or soldiers, up to sabotage, arson and ambushes. From early 1918 independent groups of Volunteers had been raiding for arms. They had long been buying or stealing individually British service rifles from individual British (often Irish) soldiers. Prior to the summer of 1920, IRA men were often using non-lethal attacks on soldiers to seize their rifles. For example:
An unsuspecting rifle patrol, with their rifles stupidly clipped on to the side of their bicycles, were surrounded in a village street by a number of young men supposedly playing a game of hurly on the village green. They gradually closed in, making friendly remarks and gestures, then knocked the soldiers off their bikes with their hurleysticks and held them up with revolvers. (Sheehan, p118)
By 1921 the IRA was suffering from a lack of manpower and resources due to internment and successful British counter-insurgency tactics. Overall strategy shifted away from the Flying Columns towards killing individuals, particularly ex-servicemen and loyalists. It doesn’t just seem to be republican anti-Protestantism, nor simply because they were suspected of being informants, but partly because of the ex-servicemen’s involvement with British forces: they were the softest target in the British system, and targeting these men kept up a form of military pressure on the British state by highlighting its failure to stabilise the security situation. By mid 1921 the persecution, kidnapping and murder of Protestants in the south had increased significantly. Loyalist civilians (e.g. Protestant farmers) were being assassinated, as were individual soldiers (unarmed and outside barracks, often in the company of local women). The IRA were also road-trenching to hinder the movement of Crown Forces – this had some effect on road transport, but by this point most British offensive operations were conducted by ‘active service’ platoons and mobile columns with air support. (Sheehan p156).
The ordinary small farmer might at any moment be called out from his house by revolver-carrying IRA to dig trenches across roads for the sabotage of police and military transport, and a day later be forced out at bayonet-point by the police and military to fill them in again. But the small Irish farmer had been in that sort of situation off and on since the days of the earliest agrarian secret societies. (Kee, p134)
Sources:
The Black and Tans, Richard Bennett, Spellmount, 1959
A Hard Local War: The British Army and the Guerrilla War in Cork 1919-1921, William Sheehan, 2011, The History Press
Ourselves Alone - The Green Flag, Volume III, Robert Kee, Penguin Books, 1972