Do ghealfadh an croí a bhí críon le cianta.
The heart would lighten that was .......weary with sadness ?

The heart, which was age-old, would delight

críon= old
le cianta = for ages

Go raibh maith agat
- and this line - Caite gan bhrí nó líonta i bpianta - I don't understand the use of the word líonta here.

(ages) spent without strength or filled with pains

Here it is "de phianta":
"Nárbh fhíor don fhile nuair a dúirt sé , ' Do ghealadh an croí a bheadh críon le cianta caite gan brí nó líonta de phianta'"
(Iníon Chíbheac by Mícheál Ua Ciarmhaic)

Go raibh maith agat arís

Hmm... I was wondering. Is cianta here the plural of the feminine noun cian - 'length of time, age'? The normal expression is leis na cianta - 'for ages'. Or is it the plural of the masculine noun cian - 'sadness, melancholy', which, according to FGB has no plural - an "uncountable" noun? But uncountable nouns are quite often pluralised in English- 'sadnesses', for example - so maybe it's the same in Irish. The poem dates from 1781 so this can easily explain any apparent deviation from the norm - and then there's "poetic licence" too...

I found two translations of it online (I'm sure there are more) - one literal in 'aislingmagazine', one non-literal and poetic in 'Trinity Journal ofLiterary Translation' - both of which translate it as "grief". The literal one has "worn out with griefs".

Seems your interpretation is right, franc

Do ghealfhadh an croí - 'The heart would gladden'
cíon - 'old; withered'

(Labhrás, "The heart, which is age-old, would delight". I don't know about German punctuation, but Do ghealfadh an croí a bhí críon le cianta in English is a "restrictive" or "defining" clause, so you can't have those two commas there. The meaning would be slightly changed.
"