I think applying theory to your problem will prove worthwhile, though a bit tedious at first. Learn how to read in open tuning, at least on the first five frets or so. Some tunes will require retuning to Dm ( DADFAD). Ligado ( hammers and pull-offs) proves very handy for speed without stress on the grace notes and fast bits. Most Celtic music can be played against a drone chord, so your thumb is keeping a beat on the low open strings while your fingers pick the melody on the high strings. Drawing up the D scale pattern on a long fretboard diagram for D tuning is really useful. Just work out where the notes from D major ( D E F# G A B C# ) occur along each string. Using this "map" and your ear, you can work out a lot of tunes pretty easily. Do a Google search for celtic "open D" to find heaps of stuff, including lots of tunes in dadgad tuning. ( D sus 4) Your last questiion is a bit confusing, and I’m not sure what you mean. You can play any melody in open tuning, but of course the fingering is different for a different tuning. I have heard some great slide guitar treatments of celtic tunes also. Exploring modes is also useful in open tunings.