Thanks everybody for the responses so far.
Rena/Girl Guide: With the lady in question returning (if it's true that she ran away) to Ireland to get married, it doesn't strike me that her running away would be 'emigration'. I would have thought that emigration would require some official paperwork, and 'running away' doesn't seem to fit. Also, on the TNA page that Rena linked to, section 4.1 'Outward passenger lists, 1890-1960' mentions that the outward passenger lists on findmypast.co.uk are for passengers leaving by ship from UK and Irish ports and
travelling to places outside Europe and the Mediterranean Sea, including the USA, Canada, India, New Zealand and Australia. So I doubt I'll have any luck there.
Several months ago I tried searching various passenger lists and got the distinct impression that there were no (or very few) records of anybody travelling to Europe, so that might be the reason. Elwyn Soutter's comment about the lack of passenger lists supports that too.
Elwyn Soutter: One of our lines of inquiry involves somebody who was rather good at circumventing rules, so there may be no passport.
A couple of questions spring to mind regarding your comments:
1 - If you didn't have to produce a birth certificate, then how would the authorities know you were actually over 21 ?
2 - Was a single woman even allowed to obtain a UK passport at that time ? I've read somewhere that in the early days of passports wives were just a footnote on their husbands passport (I didn't note the URL, but I think that may have been the USA). Apparently it wasn't until July 1928 that women aged 21+ were finally allowed to vote in the UK (
https://www.bl.uk/votes-for-women/articles/womens-suffrage-timeline )
Ramsin: With me in SE Asia, and the COVID-related restrictions at TNA I can't imagine being able to get at those records, even by proxy, for maybe another year or so.
When I first heard the 'running away to Spain' story several years ago my first thought was that it might actually be something to do with the Spanish Civil War. But we now have a more probable theory.
So I now think it's quite unlikely that we'll find any passport record or passenger list mention of her, even if she did everything according to the rules. And I doubt that she did things according to the rules.
Does anybody know whether there are any Spanish records that might be worth looking at ? Our current theory tells us where in Spain she might have gone.