"Portugal" sounds like a typical shape-note hymn name, so I searched for both and found...

In the shape-note book The Hesperian Harp, "Portugese Hymn" (pp. 378-9) is a 3-part variant of "Adeste, fideles": "Hither, ye faithful, haste with songs of triumph..."

(It also has a song named "Portugal", metrically similar to "Adeste, fideles", but the text bears no resemblance to the various translations of that carol.)

The New Oxford Book of Carols, gives much information on the history of "Adeste, fideles" (though its origin remains obscure), including this quotation of Vincent Novello, from Congregational and Chorister's Psalm and Hymn Book, 1843:

"This piece obtained the name of 'The Portuguese Hymn' from the accidental circumstance of the Duke of Leeds... having heard the hymn first performed at the Portuguese Chapel, and who, supposing it to be peculiar to the service in Portugal, introduced the melody at the Ancient Concerts, giving it the title of the 'Portuguese Hymn', by which appelation this very favourite and popular tune has ever since been distinguished;..."