An Tir has a few ways for members of the populace to declare themselves.
An ensign is a type of flag. An Tir's is described as:
Checky Or and argent, a dexter tierce sable.
which is to say: our gold and silver checkerboard, with a vertical black band (or tierce) to the side of the flag pole (the hoist side).
The overall shape of the flag is square or slightly taller than wide. The tierce is 1/4 to 1/3 of the whole width of the flag, and the checkerboard is at least three squares across and four down, with the gold in the top corner, along the tierce as shown. The back of the ensign is a mirror image of the front. The black band is always next to the flagpole.
The ensign may be adapted to a triangular pennon shape where appropriate, such as for the head of a lance, or as a small flag on top of a pavilion. Do not use the ensign as a design for a tabard, to be painted on a box, or as anything other than a flag.
Do not put the populace badge, or any other badge or arms, on the ensign.
An Tir's fieldless populace badge is described as:
(Fieldless) A lion's head erased contourny sable.
which is to say: no background, a black lion's head, with furry bits hanging down at the end of the neck, facing to the right. A few examples of ways to draw a lion's head erased are shown: the important details are that it's a lion's head, facing to the right, with the dramatically ragged edge of the neck (as opposed to a clean cut across, or some gentle curve).
Additionally, the details of the lion's features -- the teeth, the eyes, the tongue -- are not specified. In this example, the teeth are white, and his eye and tongue are red, but the actual colors of those small details is left to the artist. For example, if displayed on something red, the tongue would likely be colored blue.
Fieldless badges such as this are great for marking or stamping things, and make an excellent basis for jewelry. With that in mind, here are some suggestions for use:
Do not put the badge on the ensign.
An Tir also has an additional populace badge, which is still valid for use:
Checky Or and argent, a lion's head cabossed within a bordure sable.
In this case, we have our checkerboard with gold in top-left, with a black lion's head facing the viewer. Around the edge is a plain-lined border, also in black.
Like other fielded armory, the exact shape of the badge is up to the display. In the past, badges were submitted on circular shapes; in the present, the shapes on badge submission forms are square. A semi-triangular shape on the front of a boat would be perfectly reasonable, as would a long, rectangular strip.
Fielded badges aren't as flexible in their possibilities as fieldless, but they can see good use in enamelwork, such as for plaques on a circlet, chain, or belt. They can also lend themselves to decorating pottery, or wherever you might want a bolder statement of your affiliation, with the checky background and the bordure providing repeating frames for a lion's head. These are, of course, merely examples.
For more information on badges, see "What is an S.C.A. Heraldic Badge?".