Let me get my good friend Mathew Staunton for you...

hhp

Hi Max
You will need a lowercase b, d, g, h, m, n, and t
Best of luck to you

Mathew

Thanks so much for your kind help, Mathew and Hrant! This really does make things easier.

Just because I'm curious, and this doesn't seem like a particularly google-friendly search term… why is it that these letters sometimes appear lowercase?

BTW Max, I know you said this is supposed to be a "modern" design, but unless the client strongly objects this might actually be a nice opportunity to inject some subtle Gaelic flavor into the design - maybe restricted to those seven lc characters (with the "d", "g" and "t" possibly being the best candidates) if only in the form of alternate characters. It's the sort of cultural sensitivity that I for one value quite a bit.

hhp

Jack, in Irish orthography certain prefixed letters are not capitalised in mixed-case text; rather, the initial letter of the root word is. So, in Max's example, the mixed-case version would be 'hÉireann'. The h in this case indicates an aspirated pronunciation, presumably triggered by the preceding vowel.* In order to maintain this orthographic distinction in all-caps text, the prefixed letter conventionally either remains lowercase or, in some typography and signage, may be a smallcap. In an all-caps typeface, my inclination would be to either go for the smallcap option or ensure that the nominally lowercase forms are designed to harmonise in all-caps settings by adjusting their proportions and weight and perhaps employing bilateral serifs on ascenders.
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* I'm intrigued by the different ways in which languages evolve means to deal with successive vowel sounds across word breaks. Irish aspirates the second vowel: na hÉireann rather than na Éireann. French tends to elide the first vowel: l’école rather than la école. In spoken English, vowels are also elided quite often, but not usually in writing, and British English has the delightful insistence that aspiration sometimes isn't enough: an history rather than a history.

Once it was explained that these small letters were prefixed, I had something to try Googling with (Irish prefixed orthography). I found a page on Irish pronunciation that had many such letters in one section, on a phenomenon called eclipsis. That may be too specific, as it may not be the only reason for such letters, but it may be useful in turning up more comprehensive detail.

Hmm. Apparently eclipsis occurs a lot, and it's only when it is not expected that a prothetic consonant is added (sometimes also called a prosthetic consonant, and apparently not just when one's spelling checker alters it behind one's back - that's a legitimate alternate name).

Thanks for the concise explanation, John. And bilateral serifs is definitely an idea worth exploring.

I appreciate the suggestion, Hrant, though it's not clear that Gaelic flavor would be a plus for this particular client.