Welcome Hlynur to our board!
Thanks for your thoughts. You're certainly right about 'tobhta' originating from Icelandic/Old Norse 'tóft'. Skiplaga (with less certainty) might be connected to ON 'skipulag'. 'Sgudal' and 'slaodach' have good Celtic etymologies, although I'd think that there's a deeper relationship between 'slaodach' and 'slóð' on Indo-European level. Icelandic 'skófla' [sgoubla] doesn't seem to be a cognate of 'sgioblaich', because in Old Norse the combination of -fl- would spell as -vl- and became -bl- only in the later Icelandic period.
Just in case, in my reply I was using the following dictionaries, hope they'll be of interest to you too:
Altnordisches Etymologisches Woerterbuch by Jan de Vrieshttp://www.scribd.com/doc/78998246/Altn ... sed#scribd
An Etymological Dictionary of the Gaelic Language by Alexander MacBainhttp://www.ceantar.org/Dicts/MB2