First, a little historical diversion:
Before talking about the importance and meaning of blood rites in modern Irish witchcraft I want to say a few words about the revival of their use and significance:
Until recently we tended to raise our own domesticated animals and slaughter them for food. Animals were important particularly after the Famine. As our ancestors watched their children die, they also watched a record crop of maize, wheat, corn being harvested. They were allowed only potatos, the crops grown in such abundance were protected by RIC men from the populace, who were to die rather than touch food not intended for them....after this experience the ¼ of the populace left had two great ambitions, own land and own animals. There was affection and respect and - most importantly- gratitude for these animals such as finds its nearest echo in our neolithic ancestors.
The Blood rites had fallen into some obscurity: only the Kilkenny and Tipperary clans maintained them to any great degree. The years between Cromwell and Famine had taken their toll. The death of the three leaders of the western clans in the famine and the devastation of the populace had left a dispirited and scattered network of families across Ireland. The subsequent revival in the blood rites in this period for the reasons outlined in the paragraph above can be directly accredited with adding a new impetus to the family traditoin and pehaps even with saving it. The need for wise wimmin and parish "doctors" and unofficial vets enabled the preservation of folkcharms and remedies, and gave the practitioners a valuable place in the rural community : a a practical role, and this has set the stage for future generation from that point. Gone were the days of philosophical learning and bardic training. Now the training took place as one worked: you earned your place as you learned.
Why were blood rites so important, to the peasantry and to the practitioner? Well, the blood rites are the collection of blood motifs outlined in section one, and the peasantry, fearful of a return to the famine, (remember famine appeared on a regular basis for five years or more) searched the face of nature for any signs or portents and were fearful of alientating the spirits in any way. The anger of an ascendancy landlord over a poached rabbit was not as feared as the anger of the earth against man: having suffered her lash only a few years previously, they were determined not to alienate her again.The balance necame important once more, especially as the new political upheavels spoke to the rural population of returning the land to the native farmer.
With the emergence of the catholic mioddle class in the late 19th century, we were allowed once more to play a real role in the emergence of the modern nation, one in which old customs, old culture was revived and the irishman became proud of his heritage again. However the importance of the rural wise woman never faded and was of more real help than all the intellectualising of the rest of us. The rural practitioner bound the land to the people, and vice versa. She preserved the balance so horrendously defiled, and the blood rites, the use of sacrifice and the letting of blood, the offering of sacrifice back to the geni loci and the invokation of the elementals, all had direct direct magical and psychological effects . For example It is no surprise that blood ties of a more familial nature reawakened at this period, and almost extinct lines were revived. Also, the sacrifice of the 1916 martyrs, curiously enough preceded by Padraig Pearse's exhortation that "this old earth needed to run red with the blood of young men once more" to rejuvenate and revive her, is so breathtakingly in tune with the spirit of the blood rites as to be almost painful in its intensity and synchronicity.
The Modern use of blood in magic
Blood is still used and the blood rites still exist, but modern sensibilities have demanded a large degree of substitution. For example to evoke the sacrifice of animals, for the gods/forces of war or hunt, one can use a piece of red meat, still bloody, and squeeze the blood from it: the meat is then cooked and offered to (insert preferred choice) gods/spirits/geni loci/elementals/Noddy. For human sacrifice a blood let, often of a small drop, is used and this is substituted for such magic purpose as binding, thanksgiving, propitiary needs, divination. Another common sight is the "sacrifice" of a clay model smeared with blood in place of an animal or human. Writing in blood is usually effected by adding a few drops to an already charged ink. Emotional value placed on an object is often seen as agood substitute for the earthier relevance of ones own or another's blood.
However efficacious these methods for healing, or divination, when it comes to cursing and spell casting it is hard to find a good enough substitute for even a drop or two of blood: and the raw meat method above works extremely well in place of an actual animal life. Those who fish often kill, and cook one small fish on site, leaving the remains for the birds/predators. This custom is hard to duplicate with substitues! And is you just want that spell to work well, you can't attract magic forces better than with a life force. To overcome the disadvantages of not using blood one must develope a remarkable sense of will and strength of mind.
So there are those who substitute the power of blood with their own wills and minds, and those who feel the same power ( and they would claim more sense of connection with the natural world ) through retaining some link with the actual power of blood. As always the balance between the spiritual and esoteric and the natural worlds needs careful examination and preservation. Personally unless you find meat./blood impossibly distasteful i strongly recommend that you do not remove yourself from its use altogether. Even pricking your finger can have great effect!
One word of caution on substitutions: there is a modern tendency among wiccans to use menstrual blood as a substitute for all kinds of blood rites: the ancient use of menstrual blood was far far more circumscribed - like all powerful natural magical ingredients it should be used in its correct context, and is not suitable notr is it appropriate to use it for every kind of magic. It is used as a rule of thumb for solemn matter, for cursing, for healing in childbirth or fertility issues. to make land fertile and to to bind family members. Conversely it is traditional to avoid it in matters of sex, love, trivial spells, and bad luck to use it in spells concerned with money, or material prosperity. however these "taboos" vary widely from area to area: what most families have in common is that at least some spells are taboo for menstrual blood.