Let me just preface this by saying I'm a dough eyed optimist, but here's my two cents that no one asked for.
The League of Ireland is not the death sentence career wise that it might once have been deemed a few years ago. The strength of the league in general has gotten remarkably better and as each year ends more and more players get pretty decent moves out of it and are getting replaced ably by other league of Ireland players.
If he goes and gets the goals in, there's no dramatic big move from the Premiership but it is possible a league one or championship club could take notice.
If he hasn't cut it at the lower leagues, how does he turn this perceived negative into a positive?
The club have made a very personal and sincere attempt to get him there. When sliding down the divisions bouncing from club to club, feeling wanted boosts your confidence. The league is competitive but he's well able for it. Believe me, the Sligo Rovers fans aren't expecting fancy step overs, false 9, tikki takka shit, they just want some one to score goals. Which is just as well, because that's what he is capable of, if they get him going goals wise , he'll be adored there and no one would begrudge a move back to England if it eventuated.
Sligo itself is a beautiful spot nestled into one of the most scenic parts of Ireland, it's hard not to have a nice quality of life there if you're a young lad with a bit of money, coupled with stunning scenery each way you look. If he takes this chance seriously and has a happy time there he could reignite his career completely.
...... Or he might just do shit. Hope he goes well though.