Seeing how I'm working on a timeline with this POD, I should probably contribute:
King of Ireland is a little unrealistic IMO. Henry VIII had no desire to set up a potential rival branch of the Tudors, not with the War of the Roses just finished. However, Fitzroy will probably be sent off to Dublin at some point in his capacity as Lord-Lieutenaunt and go some military escapades, the success of which I can't determine but could well help curb the Geraldine Rebellion.
The way I see it, Fitzroy won't become heir directly. Henry VIII will probably have a son by Jane Seymour and therefore a questionable legitimisation will be unneeded. He will however, remain a favourite of the King right up to 1547 when he will no doubt be able to beat the Seymour's for control of the regency council. What's more, due to his maritial link to the Howard's the Regency of Edward VI will probably prove far more conservative in religious matters, meaning a far more stable time for England as a whole. Assuming Edward still falls sickly ITTL, Fitzroy will be a very strong position to be legitimised and declared heir to the ailing child. In 1553, we have Henry IX, a healthy young King with all the Tudor cunning needed to leave a lasting effect on the British Isles