If you previously took the Swift ferry and found it tolerable, you do not need to concern yourself with comfort. Either company's conventional ferries will be more comfortable in all but the roughest of seas than the Swift is in normal conditions.
I would personally always choose to travel with Stena Line where possible. Irish Ferries have a long history of absolutely abhorrent employment practices and a general anti-passenger attitude, something which I prefer not to support when I have an alternative. Both companies' ships are modern, comfortable vessels - Irish Ferries are more lavishly decorated, with Stena Line a little more down-to-earth - but you will certainly not be disappointed by either. Food and duty-free are cheaper on Stena Line, and they also offer free refills on coffee. Make sure to purchase any duty-free that you do not intend to consume in Ireland on your return journey, as the limits are higher in the UK.
The bus into Dublin City Centre is not included in the price of a rail and sail ticket. It costs £3 each way, and tickets must be purchased from the ferry company - they are not available on board the bus.
Finally, in case you are not aware, passengers travelling to Ireland from Great Britain are still subject to travel restrictions, and there are no plans to lift them any time soon. You are required to complete a passenger locator form and show proof of vaccination (booster required if second dose was more than 270 days before arrival date); proof of recovery (a recovery certificate in respect of a positive PCR test 11-180 days before arrival date); or proof of a negative PCR test (less than 72 hours old, NHS tests not accepted). There is no requirement to quarantine or take post-arrival tests. No restrictions whatsoever apply to return to the UK regardless of circumstances, you are treated as though you have not left the UK.
