This research aims to examine the Englyn meter in the poetry of Celtic language (Medieval Welsh) that requires the poetic texts to conform to an abstract prosodic template. This counting meter regulates the phonological constituency on the same metric level of the prosodic hierarchy rather than on the metrical hierarchy in verse (the line). In the main types of Englyn meters, Englyn milwr and Englyn penfyr, phonological units of each line are constrained with a certain number of syllables and rhyme with the final syllable of most lines. This research offers a markedness-based analysis that generates the well-formedness meter of Celtic language Welsh poetry. The Optimality Theoretical analysis derives the constraint on the phonological constituency over a certain metric level of the prosodic hierarchy (the line) with markedness constraints. Further coping constraints that are normally used for reduplication are needed to account for the rhyme of the final syllable in Englyn meter. The analysis offered supports the Development hypothesis, which is fundamental to generative metrics indicating that meter is evidently related to general language phonology. These results could help analyse other counting meters with restricted phonological constituency.