My information comes from Iain O'hAnnaidh, a very very very reliable source in Welsh (he's a native Welsh speaker, a philologist in Celtic languages and a researcher about Welsh language and culture) and I trust him completely.
However, I had press his information about Welsh names and I can copy it here:
PLACE NAMES AS FIRST NAMES Some names from toponyms (e.g. Hirwaun) are not usually used as a first Christian name. They occur as a second element in the name and indicate a connection with a particular locality. This is also the case with certain saints names (e.g. Tegla, from the village of Llandegla).
PLACE NAMES AS FORENAMES. Although not common, there are instances of place names becoming forenames. This practice dates from the 1800s, among ministers and preachers. Because so many had the same English names (David Jones, John Williams, and so on) it was necessary to add something to distinguish themselves from their namesakes (Of course, they could have changed their names completely, and given themselves Welsh names, but his would have appeared odd or lunatic in those days).
Generally, people often acquired a place-name tag to makes themselves more identifiable. It could be the place of origin, or the place in which a minister had settled and with which he had become associated. For example, to give a hypothetical example, John Jones from Aberteifi (mouth of the river Teifi) might be called by other people John Jones Aberteifi (the John Jones from Aberteifi, to distinguish him from the John Jones from Dolgellau, or Caergybi, or Aberhonddu, etc). The man himself might then style himself John Aberteifi Jones, or rather than the town name might use only the river name — John Teifi Jones. Generally, because the ministers were in the Welsh non-Conformist churches, and were Welsh-speakers and generally patriotic, they would use the Welsh form of names, rather than the alien English forms. However, there are many examples of English forms in use - as in (another hypothetical example until I find some genuine ones!) John Cardiff Williams rather than John Caerdydd Williams.
These middle names would not be official - they would not be adopted as a legal name. However, they would later be perceived as male names and in succeeding generations be used as a forename, giving curious results such as Aeron being used as the name for male - in fact Aeron was a Celtic goddess of war, regarded by the British as the river spirit of this Ceredigion river However, any memory that it was a female goddess name is lost (it is nothing more than ‘a river name’ for most people). A female forename has in fact come into use by adding -a : Aerona.
EMLYN (EMLŸN) ♂ Pronunciation: EM-lin [ˈɛmlɪn] Notes: From a district name.
The etymology coming from Aemilianus was proposed for the place name (as the composition of Gruffudd's work states, without the "also" present in other entries) but it seems that in the last years it has been desestimated:
"The name derives from am (around, on both sides of) and glyn (valley), the valley in question being presumably the Cuch.
2 ^ Charles, B. G., The Placenames of Pembrokeshire, National Library of Wales, Aberystwyth, 1992, ISBN 0-907158-58-7, p 347"
Since a lot of Welsh place names comes from given names (Latin or not) of saints or owners, the etymology for the place name Emlyn from the Latin Aemilianus implied the very possible (early) use of Emlyn as first name: use > lost of popularity > disappearance > rebirth in use. But this etymology seems to be an educated folk etymology and I don't find examples of use before 19th century. Even in SerenGomer's articles (1823) appealing to give children Welsh names, with old and new Welsh names proposed, Emlyn is not present.