We can find a few hints in mythology and ritual practices that indicate that people who intentionally stepped outside of society needed to be ritually cleansed before re-entering it. Specifically there is indications that a person who had left society to live in a wild state and who wished to re-enter society needed to be ritually cleansed using a process that featured a ritual meal, usually a broth (McCone, 1990). This process may have involved the broth being both consumed as well as asperged over the person or symbolically bathed in. This broth would have been made from food that was being ritually offered to the Gods and so was sacred by association as it were. The Fianna, who lived a portion of their time outside society, seem to have had cleansing rituals in order to re-enter society later and these rituals may have involved ritual anointing with milk or butter (PSLV, 2011). In this way we see that the food used for ritual feasting could play a role in purification, particularly the more significant or serious purifications including redeeming people who had been living wild or as outlaws.
On a simpler level we see the concept of entering a sacred place or space, especially for ritual or magical purposes, by first walking three times around the space sunwise [clockwise]. This practice has remained through the modern period in folk practices, but the concept of approaching somewhere from a sunwise direction with positive intentions, or indeed circling it against the sun for cursing, can be found in the mythology indicating the deep roots of the idea. It is a quick and basic way to draw beneficial energy, to simply walk clockwise around something.
As with many other cultures we also see the idea of burning different herbs to cleanse away baneful energy. The most well known in Irish and also Scottish culture may be juniper. Juniper is mentioned by various authors, including Danaher, Evans, and MacNeil, for its protective qualities in folk belief and for the widespread practice of burning juniper in the home and stables on the Quarter (Scottish cross-Quarter) days to be rid of dangerous energy and to bless the space and people. Another less well known herb burned for protection against evil spirits and baneful magic was mugwort, which was also kept around the home, tied onto livestock, and worn on a person's clothing for purification and to ward off fairies and witches (MacCoitir, 2006). Rosemary was also used especially as a fumigant in sick rooms, carrying the idea of cleansing away lingering illness or baneful energy in the atmosphere.
We don't know for certain exactly how the pre-Christian Irish viewed ideas like the Greek concept of miasma, if they even had such a concept themselves, but we do know that there were concepts relating to cleansing of a person, which allowed them to re-claim a place in society. Irish society in general was one that was very focused on reparitive justice, as we see in the Brehon Laws and in the mythology (Kelly, 2005). This, perhaps, explains why the focus on purification and cleansing is the way that it is with its emphasis on returning a person to a proper alignment with society and with its focus on protecting people and their places by keeping the energy there beneficial. We also know that specific actions such as entering a space sunwise had power to bless both a space and the person and could be used for cleansing. And finally there were practices relating to burning or wearing a variety of herbs, a few of which are mentioned here, with the belief that the smoke from these plants or their presence would cleanse away or ward off harmful energy and purify a space or person. This purification was important before engaging in any ritual activity or folk magic, and we see people engaging in such purification regularly - burning juniper on all the Quarter days, walking three times sunwise before entering a holy well, turning around three times before seeking a charm stone in a river.
For those seeking to work such magic or follow such a spirituality in a modern context maintaining a good habit of purification and cleansing is essential. Perhaps especially so for those who seek to walk a liminal way or who intentionally step outside society's bounds on a regular basis. The more baneful or harmful energy you may be around the more important it is to make sure you purify and cleanse often, but even if you live within society and keep on the straight and narrow (as it were) it's a good idea to at least purify and cleanse on the major holidays. I recommend a combination of the above mentioned methods, although I favor incorporating moving sunwise (or against the sun) into everything you do, with intention.
McCone, K., (1990) Pagan Past and Christian Present in Earl" Irish Literature
MacCoitir, N., (2006) Irish Wild Plants
Kelly, F., (2005) A Guide to Early Irish Law
P. Sufenas Virius Lupus (2011) The Hidden Imbolc http://www.patheos.com/blogs/pantheon/2011/02/the-hidden-imbolc/