St. Patrick's Day in London

St Patricks Day is on a Sunday this Year, so you will notice some crowds, There is a Parade, it goes from Hyde Park Corner to Whitehall, via Pall Mall. I would expect that area will be crowded, I don't think NYC or Boston type crowded though. After the parade, like in the US, the rest of the day is mainly an excuse to have multiple pints of Guinness in a pub, so the area pubs will be packed.

London is large though, once people dissipate, it just becomes regular busy London, and areas like Southwark, Tower Bridge, The Kings Cross/British Museum area will just be a normal mid-March Sunday, more crowded if the weather is sunny and mild.

There is a free entry festival in Trafalgar Square which will keep that area busy.

Here is a recent post that asked the same question. You might want to look through some of the responses there.

https://community.ricksteves.com/travel-forum/england/london-with-wife