Is there a single word meaning 'however' in Irish that you can use at the beginning of a sentence? I know I can't use áfach or ámh.

Doesn't "but" have the same meaning as "however" in that context??
So I'd say "Ach.. "

Merci bien -I think I'll go for - Ach, ar chaoi ar bith, or ach, pé scéal é. Which one do you think sounds more natural in Irish ? I was going to put - iomorra or as you find it in the old manuscripts - imorro, but I don't think that would be readily understood.

Personally I'd use "ach". But maybe my understanding of the usage of "however" is incorrect.

pé scéal é - I'd translate that as "anyhow" or as we say in Ireland "anyways".

iomorra - I don't know that word.

Also "mar sin féin"?

"Iomorra" is Old Irish. You find it all the time (spelt "immorgo", if I remember well) in the old texts, but the word has been forgotten long ago, I think.

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Iomorra/-o was used until 17th century. Then it disappeared. So it was still used in Early Modern Irish.
The last example in the RIA corpus is from 1919, Tosach na hOibre:
"Do theicheadar tríd an dúthaigh, iomorro, ar nós splainc teintrighe, ..."

And, it was used by Nicholas Williams in his translation of Alice’s Adventures in Wonderland (Eachtraí Eilíse i dTír na nIontas) in 2003 - but for archaic effect (the mouse citing the "driest thing I know/an rud is tirime dar chuala mé riamh"- It's about Conn Céadchathach instead of William the Conqueror in the original):

"Caogad iomorro laoch do chuir Tiobraide i reachtaibh ban ..."

Mar sin féin - I had forgotten that. That's a good phrase for "however".

Go raibh maith agaibh - (sorry I'm late in replying I've just come back from a week spent in Naoned/Nantes)