Even up until more modern times horse symbolism was important, and we see things like the Lair Bhan, (white mare) a person dressed up in a white sheet holding a carved horse head or skull who led a procession from house to house at Samhain. Holidays like Lughnasa prominently featured horse racing, which might be a race over a flat course or involve the riders swimming the horses across a river. An very old Irish belief was that horses had once been able to speak as humans could and that they were still able to understand people, making it important to always speak kindly to them (O hOgain, 2006). There are also a wide array of beliefs relating to Otherworldly horses like the Each Uisce and Kelpie; the movie Into the West deals with the story of an Otherworldly horse's relationship with two children in modern Ireland. It was believed that the seventh filly in a row born of the same mare (with no colts in between) was a lucky and blessed animal, called a fiorlair, a true mare (O hOgain, 2006). A true mare was naturally exempt from witchcraft and fairy enchantments, and this protection extended to her rider (Monaghan, 2004). Horses in general were lucky and would be walked over newly plowed fields, on the belief that a horse trampling freshly planted seed would make the crops grow better (O hOgain, 2006). Many protective charms and superstitions are aimed at protecting horses from the evil eye, fairy mischief and general ill health.
At least one author suggests that eating horse meat was taboo in Ireland except under rare ritual circumstances; although we know that horses were eaten in Gaul and southern England they did not seem to be considered a food animal in Ireland (Monaghan, 2004; Green, 1992). Reflecting the sacred and important place that horses had in the culture, sites in Gaul that include the remains of sacrificed horses usually also include human sacrificial remains (Green, 1992). We know that in specific cases in Ireland horses were sacrificed and eaten, in association with the crowning of a king. Ceisiwr Serith posits that horse sacrifices at ritual inaugurations are related to similar Indo-European practices, especially Vedic (Fickett-Wilbar, 2012). A ritual was enacted in Ulster, according to Gerald Cambrensis writing in the 13th century, where the new king had sex with a white mare who was then killed and stewed; the king bathes in the stew and then eats it as do the gathered people (Puuhvel, 1981). This ritual likely had ties to the horse's symbolism and represented the king joining with the goddess of sovereignty (whichever one that may have been, I suspect Macha, although killing a horse wouldn't make sense when that was the animal that may have represented her).
Although I support traditional religious animal sacrifice in a Celtic and Norse context I am absolutely against sacrificing or eating horses. This is a controversial topic, but my opinion on this is firm. At one time I had held a different view on this born, I must admit, out of a hesitance to judge modern cultures that still eat horses. But the reality is I can judge the practice as wrong - like eating whale, dog, or tiger - without condemning the entire culture that does it. The ritual recorded by Gerald is a main one used by modern people wanting to do horse sacrifices to defend the idea, however it should be obvious for several reasons why this ritual does not justify modern horse sacrifice. Firstly, it was rarely done, as far as the evidence we have shows, and only on the most significant of events, the crowning of a king and his marriage to the land. We have no modern equivalent to this. Secondly the ritual also involved public bestiality and bathing in the food before it was served; I hope the reasons not to do this is self-evident. Beyond this, as can be seen by the Gaulish examples of interred horse and human sacrifices, the killing of horses seems to have been viewed as an occasion of the utmost gravity, on par with offering a human life. Green theorizes that these events related to the fulfillment of battle pledges, where a warrior going to fight promised to give to the Gods all the spoils of war, including weapons, horses, and human captives in exchange for victory (Green, 1992). Just as we no longer practice human sacrifice because it goes against our social norms and morality, so too should we leave horse sacrifice in the past. Horses, like dogs, are animals that we have domesticated to work with us and as pets; they are not food. In the past our ancestors may have eaten them, but they also had far fewer options than we do; they needed to eat their domestic pets - we don't.
I also feel strongly that it is wrong to sacrifice horses to Macha especially. In Irish myth it is almost always geis to eat the animal that represents or is connected to you; Cu Chulain has a geis against eating dog, Dairmud has a geis not to hunt the boar that is magically bound to him, and Conaire cannot hunt birds, to give some examples. Since horses are Macha's animal it follows that killing or eating them would be offensive to her. I personallt received a geis against eating horse when I became her priestess. We do not have a single example from myth or folklore of horses being sacrificed to Macha and we do have evidence that killing or eating a symbolic animal was taboo.
There are also more direct ways to help, if you feel moved to do something in honor of horses or in the name of a horse related deity. You can donate to a horse related charity such as Equus, or find a local horse rescue in your area. A friend's uncle has been giving homes to abandoned horses for years and is now struggling to feed them - if you want to help there is a page set up for donations here. If its possible you can consider finding a local stable and taking riding lessons, or just visiting to spend some time around the animals. Getting to know horses in the real world will give you a much better understanding of their importance and sacredness in the ancient world, in my opinion.
O hOgain, D., (2006) The Lore of Ireland
Monaghan, P., (2004). Encyclopedia of Celtic Mythology and Folklore
Green, M., (1992). Animals in Celtic Life and Myth
Puuvel, J., (1981) "Aspects of Equine Functionality," in Analecta Indoeuropaea , pp. 188–189
Fickett-Wilbar, D (2012). Ritual Details of the Irish Horse Sacrifice in Betha Mholaise Daiminse, Retrieved from http://www.clarkriley.com/JIES4034web/04Fickett-Wilbar(315-343).pdf