My first guess for the above is

Tá tú go han-álainn maise.

But I suspect I am at least a bit off. Any corrections or alternate suggestions?
If context matters, I meet a person (of the desired gender), and blurt out the above.

I want to say

Tá tú an-álainn maise.

Also, although "maise" is OK, I prefer "go deimhin".

but you'll have to get more input before we know if it's right or not.

Cheers,

Tim

I don't like m(h)uise poked on the end like that.

Ó, go deimhin, is álainn an cailín tu/tánn tú go hálainn.
Ambasa gurb álainn tu.
Ambasa go bhfuil(eann) tú go hálainn.

I don't like go han-álainn
My reason for that is that there are a handful of predicative adjectives that always take go, but this usage derives ultimately from the emphatic use of go with predicative adjectives. See Peadar Ua Laoghaire's use of go fuar agus go fiain agus go bocht referring to mountains and other land features - this means "really cold, wild and miserable - make no mistake!" He wrote in one of his books on this use:

It seems there are some adjectives that you would always want to be stressed and so always have go. So go hálainn already means "really beautiful" (although possibly semantically subsequently weakened to just "beautiful"). But if it were adverbial, and means "very beautifully", then go han-álainn would be fine.
Álainn an cailín is also more emphatic.

A bit of off-topic, about the historical development of the go thingy:

I have always assumed that go with the substantive verb is derived from adverbial use (though it’s interesting that it’s restricted to ‘subjective opinion’ adjectives like go maith, go hálainn, etc.). Especially since you never(?) use this go with copula (is álainn an cailín tu and not is go hálainn…).

I’ve always thought this go appeared here because of the Old Irish usage of the verb at·tá ‘is’ (which originally meant ‘stands’, and is cognate with En. stand through PIE *steh₂-) and ní·fil ‘is not’ (which kept some the structure of original ‘see…!’) – both could only take prepositional phrases and adverbs as predicates (but not nouns and adjectives) and ní·fil also took its subject in the accusative (the man is in the house was originally literally the man stands in the house, and the man is not in the house was see no man in the house).

So I have always assumed, maybe wrongly, that the use of go maith, go hálainn after substantive verb appeared as a way to avoid using adjectives as its predicates (but rather adverbs which syntactically were ok here).

That, I’ve thought, also explained why predicative adjectives always stay in their base form instead of agreeing with the subject today, both in Ireland and Scotland – even though predicative adjectives after the copula in Old Irish were inflected to agree with the subject. My thinking was that modern predicative adjectives continue the adverbial usage after (at·)tá.

I don’t have any doubt that PUL is right in how the structure is used and understood synchronically today (or was in early 20th c.) in Munster Irish – but if I’m right here (though I’m not sure anymore!), then I wouldn’t agree that speaking about ‘turning an adjective into an adverb by prefixing go gives very little genuine information’ here (as I think this actually might be the origin of go here, thus it gives genuine information about the historical development).

So… am I very wrong here? If so, what’s the actual story behind this go and predicative adjectives staying always in the base form?

Thank you. You seem to know your stuff, so I assume you're right that it was the other way round as far as adverbial use leading to adjective use. I think your other point is a separate point. If you're sure that the substantive verb originally could never be followed by an adjective, then you must be right on that too.

But it is still remarkable that there is only a short list of adjectives that are used with go after the substantive verb. I mean - why not all adjectives?

We are left with a slightly awkward modern-day system, as you say synchronic, whereby we say: tánn tú go hálainn, where álainn on its own would not be OK. Yet for most other adjectives: táid na cnuic go fuar agus go fiain agus go bocht - where go is an intensifier.

I think your explanation covers the diachronic development well.