He's a fisherman
The 'indefinite' article isn't needed here.
Lieg mayt an hem
He is strong [lit. "Strength lies on him"]
Here, mayt indicates strength in the sense of a power or force - he can lift a lot; merely resistive strength (less likely with a human subject, but common when talking about, say, an alloy) is instead either strength or (particularly of bonds, ropes, glue, and things holding together in general) fast.
He’s weremąnn ta mi
He's my husband [lit. "He's (a) man to/for me"]
Several different translations for 'husband' are available. This is probably the most neutral, available in both colloquial and formal contexts.
However, in colloquial contexts, it would perhaps be more common to say simply he's mack ta mi ("he's my partner/spouse"), or, if his gender needed to be emphasised (or in slightly more formal contexts), he's mack were ta mi ("he's my male partner", though much more commonly heard in heterosexual partnerships than that suggests in English). In addition to small differences in formality, mack perhaps suggests more a sustained partnership, while weremąnn might be more common coming from a romantic newlywed, and would certainly sound odd coming from someone who was separated from their husband.
More formal contexts are likely to employ the words gumma and/or áfgumma. Legally, these correspond to, broadly, "common-law husband" and "legally-solemnized husband". However, in modern practice this distinction is no longer relevant, and the former is simply used as a variation on the latter - it is sometimes even spelled with an apostrophe, indicating that it is commonly (if incorrectly) viewed as an abbreviated form. Áfgumma is likely to be found in court documents, obituries, and similarly formal contexts, and as a romantic or whimsical variation in ordinary speech; weremąnn may be found in, say, government information brochures, sombre news articles, conversations with strangers and so forth, as well as swoony things written by teenagers; mack is more likely to be found in conversations with friends, lighthearted news articles* and the like.
*so, for example, in a newspaper one might expect:
Lieg she ybhydwąld av an áfgumma ay ann sune - she is survived by a husband and one son
Ląy she uð head tha twáda stuwa fąsht ay tha weremąnn - she and her husband lay trapped for two hours
She nu stá ay tha mack afta ybhywenna ta an rollovaprize! - she and her husband have now won a rollover prize!
Stá he in tha huas menn
He is in my house