This is kind of a throughout history question, but because you've named examples, we can look at some of them.

American Revolution

Here's one simple answer: the minutemen were a civilian militia that wore no uniforms. They could fight in the ordinary wool or linen coats (depending on the climate and time of year) that they might wear in everyday life.

Okay, but what you're asking about is the actual enlisted troops. Uniforms for them were a problem. The stereotypical view of the colonial era is that women produced most of the textiles used by the family at home, and everyone knows that there was a period of patriotic homespun to protest reliance on England and show American independence, but in reality the colonies depended hugely on British textile manufacture. The vast majority of fabric came from British mills - during 1769-1771, about 6.5 million yards of linen were imported. The government had no reason to support American textile manufacture during the colonial years, as the infrastructure was firmly in place back in Great Britain. Remote rural populations were largely self-sufficient, and even when fabric was purchased it was generally made up into clothing at home, but American towns and cities had tailors and seamstresses for hire. Uniforms at this time were not really a different cut from ordinary clothing, just made in specific colors, so anyone accustomed to making coats and breeches could handle it.

During the war, while the trade routes weren't completely closed (merchants' advertisements can be found promising British fabrics), the Continental Army was forced to depend on domestic products on a larger scale. While blue regimental uniforms were worn, Washington wrote to the governors of Connecticut and Rhode Island to ask for "Indian or Hunting Shirts ... design'd as a Species of Uniform, both cheap and Convenient" of New England coarse linen. (An extant one from the Washington's Headquarters State Historic Site in NY is shown at the very bottom of this page These are especially associated with riflemen, but were worn by others as they were simpler and cheaper to make than proper wool regimental uniforms, and some of the soldiers really had nothing else.

Civil War

As had been the case with the colonies, the south was hampered by textile production not happening on their ground, and like the Continental Army, they ended up with coarser and nonstandard uniforms. Wool was the best fabric for the job, as it repels water and is somewhat flame-retardant; it was expensive and imported, however, so a jean with a cotton warp was often used.

At the beginning of the war, privates' uniforms were sewed up by patriotic but non-professional women for official clothing drives. Even many of the officers, who originally did have professionally made wool uniforms, ended up in the same homespun and homemade clothes as privates, as the steadily rising costs of fabric had made their official uniforms completely unaffordable only a few years into the war.

The rhetoric about how ersatz Confederate uniforms were is, however, often overblown. Grey dyes (chosen, btw, because the color was frequently used by state militias and therefore symbolized their avoidance of perceived federal overreach) were not expensive, and rusty nails and walnut shells were not commonly resorted to to produce the proper color - although the exact shade of grey did depend on the quality of the dye, and there was a wide range achieved and used. Women were employed by the depots to mass produce uniforms in large numbers, rather than individually helping out. They lacked the standardized patterns of the north and didn't care as much about uniformity, but their actual skills and therefore the uniforms themselves improved over time.

Both the south and the north had issues due to problems with manufacturing and with the officers placed in charge of supplying the troops, as well as the geographic placement of depots. There is also photographic evidence of well-dressed Confederate officers and troops from 1863 to 1865.

(Confederate Odyssey: The George W. Wray Jr. Civil War Collection at the Atlanta History Center by Gordon Jones, 2014, is a pretty great source on Confederate material culture.)

In the 1916 Rising, not everyone would have had a full uniform but the two groups involved, the Irish Volunteers and the Irish Citizens Army, had been openly drilling for several years and had plenty of time to equip their members. The biggest problem was getting guns and this was not solved before the Rising.