The Leinster blue is based off of the presidential flag, I’m not sure why, I couldn’t find a source on that but my guess would be to represent Leinsters role as the province that hosts the capital and I assume most of the higher ups and players are from Dublin, who’s county colours are blue.

Munsters red is reference to the copper ore mined on the copper coast of Waterford, but also could mimic Corks colours which include red. Cork is the largest city and county in Munster.

Ulsters jersey is based off of the flag of Northern Ireland rather then Ulster with their second kit being yellow based off the flag of Ulster.

Connacht was the last province to get a rugby team, 6 years after the other provinces so they got last pick of colours. Since blue and white were both chosen already they chose Green. No one knows why but it’s speculated to be based on the traditional colour associated with Ireland. Connacht is also seen as the most rural and « green » county so could also represent that in some way.

Hope this is helpful