Most of the information on this page is taken from "A Social History of Ancient Ireland" by P.W. Joyce, Volume II - Chapter XXII: Dress and Personal Adornment. This chapter can be found to be partially transcribed on a web site. There was some information on cosmetics borrowed from "The Compleat Anachronist #53: Cosmetics". The descriptions relate to styles of the period. I will try to provide suggestions of modern products that will be the most useful to replicate the period look.
To replicate this look with modern cosmetics is fairly simple. There are many hair rinses, semi-permanent, or permanent colorants to brighten, lighten, or completely change a base hair color. My hair is naturally mahogany brown, and I have lightened it since high school (long before any interest in "Things Irish") to bright copper-penny red-gold. Other times I've been blonde or strawberry blonde (thanks to the joys of peroxide). Obviously, not everyone wants to make a commitment like that to change their hair color. However, some blond or red highlights, or shampoo-in temporary color can brighten most hair to give a blondish or reddish gleam.
Eyebrows can be made darker with special eyebrow color kits, brow gel, eyebrow pencil, or even black mascara combed through the brow hairs. I have found no descriptions of what the preferred shape of eyebrows would be, however tweezers were often part of female burial finds. It can be guessed that some sort of plucked or shaped brows were therefore in style.
White skin is a combination of both birth and sun exposure. However, even dark skin can be lightened a bit with some judicious application of powder or foundation. Make sure to choose a VERY sheer formula no more than a shade or two paler than the skin on your neck. If you investigate online, or in upscale department stores, or MAYBE in Asian communities, you should be able to find rice powder. Rice powder most likely wasn't available in Ireland during the fifth century, however, it would simulate the look of other powders available, and it's non-toxic.
Bold red lip and cheek color can be achieved a number of ways. Cream blush can be used to brighten the cheeks, and then the same shade patted on the lips in a thicker layer. Matte lipstick could be used in the same way (both lips and cheeks), although you should take care to dab lipstick very lightly on the cheeks to help avoid clogging pores. Some cosmetic lines market lip stains, which work just like magic markers to color the lips. While the color is difficult to remove, it won't smear, fade, transfer to cups or cheeks, etc, over the course of several days. Lip stains would be particularly useful for multi-day events or wars.
Fingernails are supposed to be reddened, but not shiny. Nail lacquer (polish) is usually too shiny to accomplish the color change. Matte nail polish is available for manicures for men - talk to a nail technician about sources. If you can find a matte polish in a rusty red or terra-cotta color, try that. Another suggestion is to take advantage of the recent craze for henna painting and use henna to color your nails. This acts as a permanent stain, so you will want to keep your nails polished in your mundane life until the color grows out (expect several months of waiting). Lipstick can be rubbed into the nails to give a pinkish or reddish tone. A final suggestion is to add refined pigment powder to something like cuticle cream or Vaseline and rub it into each nail. This should end up looking like a henna stain but wash away. You might be able to find pigment cremes to accomplish the same thing. WARNING: Both pigment powders and cremes tend to be very glittery or sparkly. You have to look for a shade that is as non-reflective as possible. Finally, You might try non-toxic permenent markers and color in each nail.
There is some debate among SCA members whether body painting with woad is actually a "period practice". A Deja News search in rec.org.sca using the words "woad" and "Caesar" will turn up at least one version of this thread. Additionally, in the monograph Origins of Invaders under the heading of "The Origins of the Brittons and Picts" you can find information on the possible mis-translation of Caesar's "Gallic Wars" which is supposed to be the main support for using woad designs.
Understanding that many people are on the "pro-woad" side of the fence, I will present some basic information here.
Woad dye is an extract from the woad plant. The chemical colorant is the same as in the indigo plant (however, indigo is more concentrated, giving you more "blue" per plant). It can be found through natural dye suppliers, herbalists, at large SCA events, etc.
The "period" way to make woad body paint is to mix woad with stale human urine. When I was at my first Pennsic War and learning about woad, I heard several theories about just what sort of urine to use. These ranged from collecting the morning batch from a pregnant woman to various ages of pre-pubescent boys. I suppose this has something to do with various hormonal balances that are supposed to make the woad "stay" longer. In any case, no one I saw was actually using ANY kind of urine. Grain alcohol or vodka is the mixer of choice for modern re-enactors and SCAdians. It blends well, evaporates, doesn't leave a strong odor behind, and is *much* more safe and hygenic than decomposing bowls of pee :).
Always experiment first...never leave woad-painting until ten minutes before the big event you want to show off in! You need a chance to see how the woad dye shows up against your skin, how long it lasts, and possible changes to affect these two characteristics. Mix the woad powder with alcohol until it is a consistency you are comfortable painting on your skin (it should be thick while still somewhat runny - think the consistency of heavy cream). Then, use a paintbrush to paint and decorate to your heart's content (or, ask a friend to). Let it dry completely. Blot off any flaky residue. It should stain your skin for at least several days to weeks (if you have a job with a strict dress code, I wouldn't suggest applying it where the Boss will see it Monday :( ). You could also try adding a few drops of refined ammonia to see if it darkens the color (be careful, too much ammonia is dangerous when applied to the skin). Woad paint will stain your clothes, so be careful what you wear when being painted.
A cheap (and easy-to-clean) substitute for woad dye is a navy blue eyeliner stick. You can draw and paint designs that will wash off easily. However, these designs will also smear if touched or rubbed, so, you might want to look for some sort of clear "fixer". This can be found in body painting/tattoo kits, and as part of some drugstore or department store cosmetic lines. Lipstick fixer is the most common product, but, I have seen formulations for eyeliner as well.
Be sure to wear a strong sunscreen if you're going to be outside as you won't tan through the body paint or eyeliner. This warning applies to most sorts of body paint, even things like henna. It will leave white "ghost marks" of the design on your skin when you clean it, or it fades naturally.
Sources of Designs:
A quick method of making transfers requires gel carbon paper and onionskin tracing paper. Just put the blue side of the paper up under the the tracing paper which you have transferred the design onto once already. When you redraw the lines this makes a negative (reverse) image of the original art. This means you have to make sure it's facing opposite the way you want it to :). Oil your skin very lighly (you might try Mehlabiya oil, which is popular for henna work) and then press the transfer into place. Carbon markings will stick to the oil and leave a clear image for you to follow with your woad paint.