You're right it's not in any of the Gaelic dictionaries. But if you do a
Google search on dìlseach (the corresponding singular) you'll find a few
references, amongst others a BBC glossary where it is given as "loyalist".
Dineen has: dílseach adj faithful, devoted, forfeited, guilty, deserving of;
dílseachd báis deserving of death sm (=masculine noun) a guilty person.
So the word seems to have a "history" which may just have slipped through
the lexicogrphers' net or has become obsolete in modern Gaelic.
This leads me to believe it's a modern backformation from dìlseachd (or a
related word) to supply a Gaelic word for "loyalist".
Am Mìcheal Eile