Mae'n is the squished form of Mae hi'n.

Gareth King says in Modern Welsh: A Comprehensive Grammar:

Deg has an alternative form deng which generally appears before time-words beginning with m-: deng munud ten minutes, deng mis ten months, deng nmlynedd (NM of blynedd) ten years; and by the same token in the numerals deng mil ten thousand and deng miliwn ten million. Note also deng ⁿniwrnod (NM of diwrnod) ten days, deng milltir ten miles and deng modfedd ten inches. In most other cases, deg o° is the preferred construction – deg o °fapiau ten maps.

(ⁿ and ° indicate mutations.)

From what I can tell, it's used before vowels, but not before consonants.

But it is possible that I'm not reading Wiktionary right on this one.

It's a weird form that only shows up in certain sentences, and I can't for the life of me remember what the rule is.