As far as we know, Gustav Adolf had two Scots-Irish regiment raised by Alexander Forbes in 1630-1631 : Forbes' own and Frederick Hamilton's. These units entered Swedish service in 1631 and their whereabouts become difficult to track down after 1633. I don't think there's any information around regarding their flags, which are most likely unknown. Some Scottish units are known to have used the cross of St Andrew over a field of varying colours (notably blue and green) and Forbes' regiments may well have carried similar colours, but that is purely conjectural.
Oh, and at the risk of being a killjoy, Scots and Scots/Irish mercenaries in Danish or Swedish employ did not wear their traditional celtic garb in Germany. Plaid and tartan were regarded as primitive and tribal apparel and regimental commanders were anxious to have their men clothed in 'proper' continental clothing as soon as they landed in Denmark or Germany. By contract, the men were transported to continental Europe while sill wearing their native clothes but they were supposed to be issued with a full suit of clothing shortly after arriving at their destination. Due to occasional supply shortages, some men had to keep on wearing their celtic clothes for weeks or months before they received proper uniforms but the idea was always to ensure that Scots and Irish mercenaries looked exactly like their German colleagues.