Okay, not really Celtic, but it reminded me of something along those lines (artist license and all).
I wanted to use something simple and fluid looking, so I thought I should use a shot of the incense smoke as it has that fluid feel to it. The rest was up to Lightroom and Photoshop. Below is the original photo and the editing steps used. Please feel free to ask me questions if you have any...
Photoshop
Paint black over paler areas of smoke and darken background
Duplicate background
Flip horizontally
Difference
Duplicate background
Flip vertically
Difference
Duplicate background
Flip 90 degrees CW
Difference
50% glamour filter
Layer mask
Unsharp mask in selected areas to bring out detail
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