Tiling over old plaster

Hi, I'm re tiling the bathroom of my 1900 house, the skimmed layer of plaster beneath is coming off with the tiles for at least a section, and it has this pink plaster underneath. I've put some pictures up. It's boards, seem a foot high and maybe three long, horizontal, with this lattice pattern on them. I'm taking some more off later. Question is, if it all comes off like this, taking the top layer of old plaster off, can I tile straight onto this pink stuff? Should I have it skimmed first? It has some bits of straw in it like strammit (as I've read) but it's not full of it. Any help, advice etc greatly appreciated.

Personaly I would have it skimmed first. A lot less trouble and probably do a better job of the tiling