So reads a sign at Llyn Cerrig Bach, a small lake on the Welsh isle of Anglesey or Môn.

photo courtesy Kristoffer Hughes
Porth “door, gate”, related to portal; i “to”; (y)r “the” byd “world”; arall “other, another”.
(Incidentally, one of the best online Welsh dictionaries is maintained by the Prifysgol Cymru/University of Wales.)
How to find and pass through such a gate?
In addition to the photo above, Welsh Druid chief and author Kristoffer Hughes [Facebook link / Voices of Modern Druidry entry] offers this bilingual triad on his Facebook page:
Dyma dri o roddion Môn,
Traed y Derwyddon ar y Tir,
Cylch tragwyddol y Môr,
Coleuni diderfyn yr Awyr.
The three gifts of Môn,
The feet of the Druids upon the Land,
The eternal circle of the Sea,
The Sky’s unbounded illumination.
Here are the Three Elements of Earth, Sea and Sky — Tir, Môr and Awyr. The Welsh names work very well all by themselves as a chant and prayer: tir [teer], môr [mohr] and awyr [ah-weer].
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In 1942, a hoard of some 150 objects was discovered near the end of the lake, apparently deposited there as votive offerings. Among them is this splendid bronze plaque, now held in the National Museum of Wales in Cardiff:

crescent plaque / Wikipedia
How do we find and pass through portals to the Otherworld?
I invoke the Three Gifts of Môn,
and seek entrance to an Otherworld,
to where it is right and fitting for me to journey.
Feet of Druids, guide my steps.
True return I seek, for I have been there before,
not merely in dream and vision, and in desire,
but fully, born out of it into this life,
in the eternal circle of the Sea,
returning to it after time and times have ended here.
In this air I make the sacred sign
[with the forefinger of your dominant hand,
draw an infinity symbol in the air].
By the power of Earth, Sea and Sky,
assist me to make the journey anew,
and recall what I discover there,
so that I may share it after
for the good of the whole.
Recording the experience, whatever comes, is a valuable tool for making any subsequent journeys, and as a landmark of our practice. By making a record, I learn how I journey, which may be very different from the path others take. It may be that I recall with different senses active. Some see, but others hear, or touch, or return with no distinct impressions until they lift a musical instrument, or write a poem, or paint or draw. The more different kinds of outlets we provide in our lives and practice, the more the Otherworld can touch us here, and we can locate and recognize and draw on its inspiration.
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For more information, and images of other signposts like the “Gate to the Otherworld” mapping the region, visit “Anglesey Visualizations installed” at Monumental UK.
For an 8-minute Youtube interview with Kristoffer Hughes about the history of Druids in Wales, and their shamanic background, go here.