If you're planning to visit the village & gardens of Portmeirion, I'd certainly recommend a pre-visit lunch-time stop at Castell Deudraeth. Although the 2-course meal is a little steep at £17.95 per person, it does come with the offer of a "free" ticket to visit to Portmeirion after your meal (worth £10 per adult). Although the restaurant's biggest trump card is the beautiful building and garden, the choice of starters, main meals & desserts is quite wide and the food is generally very good & presented very nicely.
As noted by another recent reviewer, the at-table service can be a little inattentive & laboured given the apparent size of the serving team but this can just about be forgiven given the other positives I've noted. A children's 2 course meal is also reasonably good value at £7.95.